PHGY 209 Midterm Immunology
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Questions and Answers

What is the primary function of basophils in the immune response?

  • Release histamine and trigger allergic responses (correct)
  • Engulf and digest pathogens
  • Destroy cancer cells
  • Phagocytize foreign invaders and self-destruct

    • What outcome might result from inappropriate activation of the immune system?

  • Improved phagocytosis of invaders
  • Enhanced response to vaccinations
  • Increased production of antibodies
  • Autoimmune responses where the body attacks its own tissues (correct)

    • What role do neutrophils play in the immune response?

  • Release histamine to initiate inflammation
  • Produce antibodies against allergens
  • Differentiate into macrophages to capture invaders
  • Phagocytize foreign material and self-destruct (correct)

    • What type of immune cell develops into macrophages and dendritic cells?

    <p>Monocytes</p> Signup and view all the answers

    What is the primary role of eosinophils in the immune system?

    <p>Destroy parasites</p> Signup and view all the answers

    Which immune cells originate from myeloid stem cells?

    <p>Monocytes and eosinophils</p> Signup and view all the answers

    What type of immune response do mast cells primarily facilitate?

    <p>Release of histamine to signal invaders</p> Signup and view all the answers

    What transformation do monocytes undergo to combat invaders?

    <p>They transform into macrophages and dendritic cells</p> Signup and view all the answers

    What unintended effect might result from mast cell activation?

    <p>Triggering allergic reactions</p> Signup and view all the answers

    What is the primary role of B cells in adaptive immunity?

    <p>Produce antibodies in response to specific antigens</p> Signup and view all the answers

    Which type of T cell is primarily involved in the destruction of infected cells?

    <p>Cytotoxic T cells (CD8+)</p> Signup and view all the answers

    What function do Natural Killer Cells (NKC) serve in the immune system?

    <p>Detect abnormal metabolism in the body's cells</p> Signup and view all the answers

    How does adaptive immunity respond after the first exposure to an antigen?

    <p>The response becomes faster and larger upon subsequent exposures.</p> Signup and view all the answers

    What role do cytokines play in the function of T cells?

    <p>They facilitate the destruction of infected cells.</p> Signup and view all the answers

    Which statement accurately describes the differentiation of B cells?

    <p>B cells differentiate into plasma cells that secrete antibodies.</p> Signup and view all the answers

    Which type of lymphoid organ is mainly responsible for the activation of B and NK cells?

    <p>Peripheral lymphoid organs</p> Signup and view all the answers

    What happens to B cells during an immune response?

    <p>They differentiate into plasma cells that produce antibodies.</p> Signup and view all the answers

    Where do T cells develop in the immune system?

    <p>Thymus</p> Signup and view all the answers

    What function do mast cells primarily serve in immune response?

    <p>Inflammatory response in tissues</p> Signup and view all the answers

    Which lymph organ is the primary site where B cells mature?

    <p>Bone marrow</p> Signup and view all the answers

    What is the primary function of the spleen in the lymphatic system?

    <p>Phagocytosis of microbes</p> Signup and view all the answers

    Which structure is primarily involved in the filtration of lymphatic fluid?

    <p>Lymph nodes</p> Signup and view all the answers

    What happens to the thymus as an individual ages?

    <p>It atrophies and its function declines</p> Signup and view all the answers

    Which components are primarily found in the thymus?

    <p>Epithelial cells and dendritic cells</p> Signup and view all the answers

    What is the role of tight junctions between epithelial cells in the immune system?

    <p>To create a physical barrier preventing pathogen entry</p> Signup and view all the answers

    Which secretion creates a hostile environment for invaders in the digestive tract?

    <p>Gastric juices</p> Signup and view all the answers

    How does the skin contribute to innate immunity?

    <p>By forming a continuous water-resistant barrier</p> Signup and view all the answers

    What role do symbiotic flora play in the immune system?

    <p>They create competition for resources to inhibit new invaders</p> Signup and view all the answers

    What is the purpose of mucus in the innate immune response?

    <p>To trap foreign particles for removal</p> Signup and view all the answers

    What is the role of Type 1 interferons in the immune response?

    <p>To inhibit viral replication</p> Signup and view all the answers

    Which component is responsible for tagging pathogens for phagocytosis?

    <p>C3b</p> Signup and view all the answers

    What is the primary function of Fe-binding proteins in the immune system?

    <p>To restrict availability of iron to limit pathogen growth</p> Signup and view all the answers

    Which type of immune cells acts as phagocytes in the immune system?

    <p>Neutrophils</p> Signup and view all the answers

    Which of the following describes the functions of inflammatory mediator cells?

    <p>To initiate and regulate inflammation responses</p> Signup and view all the answers

    What is the first stage of the inflammatory response?

    <p>Vasodilation</p> Signup and view all the answers

    During which stage do phagocytic cells enter the damaged tissue?

    <p>Emigration of phagocytes</p> Signup and view all the answers

    What immediate effect does vasodilation have during the inflammatory response?

    <p>Increased blood flow to bring immune cells</p> Signup and view all the answers

    What is the primary function of the tissue repair stage in the inflammatory response?

    <p>To restore normal tissue function</p> Signup and view all the answers

    Which of the following correctly describes the sequence of stages in the inflammatory response?

    <p>Vasodilation, Emigration of phagocytes, Tissue Repair</p> Signup and view all the answers

    What is the primary role of MHC Class I molecules in immune response?

    <p>To display endogenous antigens for detection by CD8+ T cells</p> Signup and view all the answers

    Which statement accurately describes the role of Natural Killer (NK) cells?

    <p>NK cells release perforin and granzyme to induce apoptosis in target cells</p> Signup and view all the answers

    What occurs when MHC Class I molecules are lacking on a cell?

    <p>Natural Killer (NK) cells will induce death of the cell</p> Signup and view all the answers

    How do NK cells differ from cytotoxic T cells in their mechanism of action?

    <p>NK cells do not recognize specific antigens like cytotoxic T cells</p> Signup and view all the answers

    What pathway is described by the process where MHC I molecules present endogenous proteins?

    <p>Endogenous pathway</p> Signup and view all the answers

    What is the role of antigen-presenting cells (APCs) in the immune system?

    <p>They phagocytose infectious agents and display their antigens.</p> Signup and view all the answers

    Which statement accurately describes the exogenous pathway in immune activation?

    <p>It includes the presentation of antigens from phagocytosed agents to CD4+ T cells.</p> Signup and view all the answers

    Which of the following HLA molecules are classified under MHC Class II?

    <p>HLA-DP, HLA-DQ, and HLA-DR</p> Signup and view all the answers

    What type of cells utilize the MHC Class II molecules for antigen presentation?

    <p>Antigen-presenting cells like macrophages and dendritic cells</p> Signup and view all the answers

    What is primarily activated by the detection of antigens displayed on MHC Class II molecules?

    <p>Helper T cells (CD4+)</p> Signup and view all the answers

    What happens to neutrophils during the process of killing infectious bacteria?

    <p>They die and contribute to the formation of NETs.</p> Signup and view all the answers

    How do chemokines function in the immune response?

    <p>They attract neutrophils to infection sites.</p> Signup and view all the answers

    What are PAMPs and what is their significance in the immune system?

    <p>They are unique structures on pathogens recognized by immune cells.</p> Signup and view all the answers

    What is the primary functional role of monocytes in the immune system?

    <p>To develop into macrophages and dendritic cells.</p> Signup and view all the answers

    Which receptors enable macrophages to recognize pathogens through PAMPs?

    <p>Pattern recognition receptors.</p> Signup and view all the answers

    What characteristic of an antigen allows it to provoke an immune response?

    <p>Immunogenicity</p> Signup and view all the answers

    What is the role of memory B cells in the immune system?

    <p>Store antibody designs for future infections</p> Signup and view all the answers

    Which stage of the adaptive immune response involves the activation of lymphocytes?

    <p>Lymphocyte activation</p> Signup and view all the answers

    What happens to B cells after they recognize an antigen?

    <p>They differentiate into plasma cells to produce antibodies</p> Signup and view all the answers

    Which feature makes antibodies effective against specific antigens?

    <p>Their specific binding to epitopes</p> Signup and view all the answers

    What is the primary role of T cells in the immune system?

    <p>Directly monitoring body cells for infection and antigens</p> Signup and view all the answers

    Which type of T cell is responsible for identifying infected body cells?

    <p>Cytotoxic T cells</p> Signup and view all the answers

    How do cytotoxic T cells recognize infected cells?

    <p>Through antibodies attached to their membranes</p> Signup and view all the answers

    What are MHC I molecules primarily responsible for in T cell activation?

    <p>Presenting endogenous antigens to cytotoxic T cells</p> Signup and view all the answers

    In relation to T cells, what is the significance of antibody presence on their membranes?

    <p>Assists in antigen recognition and monitoring of cells</p> Signup and view all the answers

    What is required for T cell activation besides binding to the antigen presented on MHC Class II?

    <p>Co-reception of CD28 and B7</p> Signup and view all the answers

    What role do dendritic cells serve in the immune system?

    <p>Phagocytose pathogens and present antigens</p> Signup and view all the answers

    Which cytokines are released from antigen-presenting cells to promote T cell activation?

    <p>IL-1 and TNF</p> Signup and view all the answers

    What is the initial event that leads to T cell activation?

    <p>Binding of T cell receptor to antigen on MHC class II</p> Signup and view all the answers

    Which statement correctly describes the role of Helper T cells during the immune response?

    <p>They stimulate the production of other B and T cells.</p> Signup and view all the answers

    What is the role of CTLA-4 in immune regulation?

    <p>Inactivates T cells by displacing CD28 from B7</p> Signup and view all the answers

    Which mechanism is primarily responsible for blocking T cell activation?

    <p>Increased levels of CTLA-4 or PD-1 by cells</p> Signup and view all the answers

    What is a consequence of PD-1 expression on T cells?

    <p>Inactivation of T cells</p> Signup and view all the answers

    Which statement correctly describes checkpoint inhibition?

    <p>It provides a necessary control mechanism to halt immune activation.</p> Signup and view all the answers

    How do regulatory T cells contribute to immune system control?

    <p>By expressing CTLA-4, which displaces CD28 from B7</p> Signup and view all the answers

    What is the effect of adding PD-1 antibodies to T cells?

    <p>They leave T cells activated by blocking PD-1 activity.</p> Signup and view all the answers

    What is the role of CLA-4 antibodies in T cell activation?

    <p>They prolong T cell activation indefinitely.</p> Signup and view all the answers

    How does the addition of PD-L1 antibodies affect T cell function?

    <p>They prevent the binding of PD-1, keeping T cells active.</p> Signup and view all the answers

    What is the consequence of using PD-1 antibodies in therapy?

    <p>T cells will be activated and may trigger cell death.</p> Signup and view all the answers

    What happens to T cells when both PD-1 and CLA-4 antibodies are introduced?

    <p>T cells remain activated indefinitely.</p> Signup and view all the answers

    What is the primary function of the variable region of an antibody?

    <p>Binds specifically to antigens</p> Signup and view all the answers

    Which statement correctly describes the constant region of an immunoglobulin?

    <p>It consists of the fragment that crystallizes</p> Signup and view all the answers

    Which component of an antibody is essential for its classification into different immunoglobulin types?

    <p>Variable region</p> Signup and view all the answers

    What are the primary chains that make up an antibody structure?

    <p>Two heavy and two light chains</p> Signup and view all the answers

    Which part of the antibody is referred to as the antigen-binding fragment?

    <p>Fab region</p> Signup and view all the answers

    Which immunoglobulin class is primarily responsible for providing passive immunity to an infant through the placenta?

    <p>IgG</p> Signup and view all the answers

    What characteristic differentiates IgM from other immunoglobulin classes?

    <p>It triggers complement activation.</p> Signup and view all the answers

    Which immunoglobulin class is mainly found in mucus-associated lymphoid tissue and certain bodily secretions?

    <p>IgA</p> Signup and view all the answers

    Which immunoglobulin is produced prenatally and may not have a significant role in immune defense postnatally?

    <p>IgD</p> Signup and view all the answers

    What is the structural characteristic of IgA that enables it to be effective in mucosal immunity?

    <p>Dimer with 4 binding sites</p> Signup and view all the answers

    What is the primary function of opsonization in the immune response?

    <p>To tag pathogens for elimination by phagocytes</p> Signup and view all the answers

    What is the result of the complement system being activated by the binding of an antibody to an antigen?

    <p>Formation of the membrane attack complex leading to cell lysis</p> Signup and view all the answers

    Which process involves the clustering together of antigens for easier phagocytosis?

    <p>Agglutination or precipitation</p> Signup and view all the answers

    How do cytotoxic T cells contribute to antibody-dependent cellular toxicity?

    <p>By provoking cell death when specific antibodies are bound to cell membranes</p> Signup and view all the answers

    What role does neutralization play in the immune response?

    <p>It prevents a bacterium from releasing toxins</p> Signup and view all the answers

    What is the main characteristic of active immunity?

    <p>It creates memory B cells and provides long-lasting protection.</p> Signup and view all the answers

    Which type of immunity is achieved through exposure to a vaccine?

    <p>Artificial immunity</p> Signup and view all the answers

    What elements are typically included in a vaccine?

    <p>Dead or disarmed pathogens and harmless antigenic molecules</p> Signup and view all the answers

    Natural immunity is primarily characterized by which of the following?

    <p>Chance infection by pathogens such as viruses</p> Signup and view all the answers

    What type of antigen exposure do vaccines use to stimulate immunity?

    <p>Single antigen proteins or harmless components from pathogens</p> Signup and view all the answers

    What is a characteristic of passive immunity?

    <p>It provides temporary protection through direct antibody transfer.</p> Signup and view all the answers

    During an immune response, what are plasma B cells primarily responsible for?

    <p>Secreting antibodies to fight the infection.</p> Signup and view all the answers

    What happens to the serum memory B cells over time?

    <p>They decline for the specific infection.</p> Signup and view all the answers

    What is the role of a booster vaccination?

    <p>It re-initiates activation of memory B cells.</p> Signup and view all the answers

    Which mechanism explains natural passive immunity?

    <p>Antibodies transferred via breast milk.</p> Signup and view all the answers

    What is the function of RAGs in B-cell development?

    <p>They splice out unused segments of DNA during receptor development.</p> Signup and view all the answers

    How does Tdt contribute to B-cell diversity?

    <p>By introducing frame-shift mutations through nucleotide insertion.</p> Signup and view all the answers

    What occurs during class switching in B cells?

    <p>B cells replace the heavy chain with a different type while keeping the variable region.</p> Signup and view all the answers

    What defines the transition from Pre-B cell to mature B cell?

    <p>The ability to bind to specific antigens.</p> Signup and view all the answers

    What is an initial characteristic of Pre-B cells?

    <p>They express RAGs for heavy chain recombination.</p> Signup and view all the answers

    What characterizes the primary immune response to a pathogen?

    <p>Time is required to form antibodies against the specific pathogen.</p> Signup and view all the answers

    Which statement accurately describes the secondary immune response?

    <p>It leads to a quicker and more substantial antibody production.</p> Signup and view all the answers

    Why might an individual not notice an infection during a secondary immune response?

    <p>The rapid antibody production minimizes the severity of the infection.</p> Signup and view all the answers

    What is a key difference between the primary and secondary immune responses?

    <p>The secondary response can often occur without noticeable symptoms.</p> Signup and view all the answers

    What is the primary reason the body does not create antibodies against self-antigens?

    <p>To prevent auto-immune responses</p> Signup and view all the answers

    What percentage of T cells are typically eliminated in the thymus through selection processes?

    <p>95%</p> Signup and view all the answers

    Which statement accurately describes the fate of T cells that cannot recognize MHC Class II molecules?

    <p>They are negatively selected and destroyed</p> Signup and view all the answers

    What characterizes T cells that are negatively selected in reference to MHC Class I molecules?

    <p>They are destroyed if they attempt to recognize self-antigens</p> Signup and view all the answers

    What mechanism is primarily involved in developing immune tolerance during fetal or post-natal life?

    <p>Clonal inactivation and apoptosis of specific immune cells</p> Signup and view all the answers

    What do cytotoxic T cells release to induce cell death in infected cells?

    <p>Perforin and granzymes</p> Signup and view all the answers

    What indicates that a cell is infected and prompts the action of cytotoxic T cells?

    <p>MHC I receptor displaying antigens</p> Signup and view all the answers

    What happens to the membrane of a virus-infected cell as a result of cytokine release from cytotoxic T cells?

    <p>It develops pores leading to cell lysis</p> Signup and view all the answers

    What is the primary receptor that cytotoxic T cells recognize on infected cells?

    <p>MHC I receptor</p> Signup and view all the answers

    What is the result of the binding between a cytotoxic T cell's CD8+ receptor and the MHC I of an infected cell?

    <p>Release of cytotoxic substances</p> Signup and view all the answers

    What is the greatest worldwide contributor to altered infection resistance?

    <p>Protein or calorie malnutrition</p> Signup and view all the answers

    Which factor can decrease the immune response when infected with one agent?

    <p>Pre-existing disease</p> Signup and view all the answers

    How does sleep deprivation affect the immune system?

    <p>It is associated with decreased immune function.</p> Signup and view all the answers

    Which of the following is an effect of maintaining modest exercise and physical conditioning on immunity?

    <p>It is associated with better-performing immune systems.</p> Signup and view all the answers

    Which factor negatively influences the body's resistance to cancer and infections?

    <p>Stress and mindset</p> Signup and view all the answers

    What triggers the immune T cells to target graft cells for destruction after an organ transplant?

    <p>MHC-II proteins on macrophages in graft tissue</p> Signup and view all the answers

    Which of the following is a method to avoid rejection of tissue grafts?

    <p>Administering medications that kill dividing lymphocytes</p> Signup and view all the answers

    Which drug is specifically used to block cytokine production from T cells in order to prevent tissue rejection?

    <p>Cyclosporine</p> Signup and view all the answers

    What is the result of MHC proteins being different between graft cells and the recipient's tissues?

    <p>Activation of T cells to attack graft cells</p> Signup and view all the answers

    What type of immune cells are primarily targeted by drugs that decrease the T cell population in an organ transplant recipient?

    <p>Activated T cells</p> Signup and view all the answers

    What is a potential consequence if an Rh- mother has a second Rh+ child after previously carrying an Rh+ fetus?

    <p>The fetus may experience destruction of its red blood cells.</p> Signup and view all the answers

    Which mechanism is employed to prevent an immune response in an Rh- mother after exposure to Rh+ blood?

    <p>Providing anti-Rh antibodies to neutralize any existing antibodies.</p> Signup and view all the answers

    What type of response is characterized by immediate hypersensitivity after allergen exposure?

    <p>Anaphylactic reaction.</p> Signup and view all the answers

    Which process describes how allergens trigger allergic reactions in the immune system?

    <p>Allergens activate mast cells, leading to the release of histamines.</p> Signup and view all the answers

    What role do memory B cells play in allergic responses?

    <p>They store information for a faster response to future allergen exposure.</p> Signup and view all the answers

    What is a potential consequence of excessive inflammatory response during anaphylaxis?

    <p>Systemic hypertension and bronchial constriction</p> Signup and view all the answers

    What triggers an inappropriate immune attack in autoimmune diseases?

    <p>Body proteins acting as antigens</p> Signup and view all the answers

    Which autoimmune disease is characterized by the destruction of insulin-producing cells?

    <p>Type I diabetes</p> Signup and view all the answers

    Which organ is responsible for the maturation of T cells in the immune system?

    <p>Thymus</p> Signup and view all the answers

    What is the main function of secondary lymphoid organs?

    <p>Activation of lymphocytes in response to antigens (where the immune response occurs)</p> Signup and view all the answers

    Which of the following is NOT considered a secondary lymphoid organ?

    <p>Thymus</p> Signup and view all the answers

    What is the role of Type 1 interferons in the immune response?

    <p>Inhibit viral replication</p> Signup and view all the answers

    Which of the following is a function of Fe-binding proteins in the immune system?

    <p>restricting pathogen growth</p> Signup and view all the answers

    Which immune cells are primarily responsible for engulfing pathogens?

    <p>Phagocytic cells</p> Signup and view all the answers

    What mechanism describes the tagging of pathogens for phagocytosis by the complement system?

    <p>Opsonization</p> Signup and view all the answers

    Which of the following cells is categorized as an inflammatory mediator?

    <p>Basophils</p> Signup and view all the answers

    Study Notes

    Immune System

    • The immune system is a network of cells and organs that defends the body against pathogens and abnormal cell growth.
    • The immune system relies on blood and lymph to connect its cells and tissues.
    • Immune system problems can lead to autoimmune diseases where the body attacks itself.

    Innate Immune System

    • The innate immune system is a non-specific defense mechanism that is always active.
    • It relies on specialized white blood cells called granulocytes and agranulocytes.

    Granulocytes

    • Basophils release histamine and prostaglandins, triggering allergic reactions and attacking pathogens.
    • Eosinophils specialize in destroying parasites.
    • Neutrophils are phagocytic cells that engulf and destroy invaders, resulting in pus formation.
    • Mast Cells release histamine and other chemicals, acting as an alarm system for the presence of invaders.

    Agranulocytes

    • Monocytes develop into macrophages and dendritic cells, which are phagocytic cells that capture and destroy invaders.

    White Blood Cells

    • Eosinophils are white blood cells that kill parasites
    • Agranulocytes are white blood cells that develop from myeloid stem cells
    • Monocytes are agranulocytes that develop into macrophages and dendritic cells
    • Macrophages and dendritic cells perform phagocytosis, capturing and destroying invaders
    • Mast cells are immune cells that release histamine and other chemicals when invaders or parasites are present
    • Mast cells can accidentally trigger allergic reactions

    Adaptive Immunity

    • Immune response specific to invading pathogens
    • Faster and stronger response to previously encountered pathogens
    • Leukocytes (white blood cells) from lymphoid stem cells

    B Cells

    • Produce antibodies (proteins) that recognize and neutralize specific antigens (foreign substances)

    T Cells

    • Produce antibodies that bind to receptors on the cell membrane
    • Recognize infected cells and trigger their destruction using cytokines (signaling molecules)

    Types of T Cells

    • Cytotoxic T cells (CD8+): Directly kill infected cells
    • Helper T cells (CD4+): Activate other immune cells
    • Regulatory T cells (CD4+): Suppress immune responses to prevent autoimmune reactions

    Natural Killer Cells (NKC)

    • Detect abnormal metabolic activity in cells
    • Target and destroy cells exhibiting suspicious metabolism (e.g., tumor cells, foreign tissue, infected cells)

    Immune System Tissues

    • Primary lymphoid organs are where immune cells develop from stem cells.
    • Secondary lymphoid organs are where immune responses occur.
    • The bone marrow is the primary site of hematopoiesis, where all immune cells are made.
    • Basophils, eosinophils, neutrophils, mast cells, monocytes, B cells, and NK cells are all produced in the bone marrow.
    • B cells differentiate into plasma cells in the peripheral lymphoid organs.
    • Plasma cells secrete antibodies during immune responses to fight infections.
    • Mast cells reside in almost all tissues after being produced in the bone marrow and play a role in inflammatory responses.
    • T cells are produced in the thymus, a gland located above the heart.
    • B and NK cells are activated in the peripheral lymphoid organs.

    Immune System Tissues

    • The immune system consists of primary and secondary tissues.
    • Primary tissues are where stem cells divide and immune cells develop.
    • Secondary tissues are where the immune response occurs.

    Lymphoid Organs

    • Lymphoid organs are important components of the immune system.
    • Bone marrow is the site of hematopoiesis, where all immune cells are made.
    • B cells and natural killer (NK) cells are initially made in the bone marrow and then activated in peripheral lymphoid organs.

    Immune Cells

    • Basophils, eosinophils, neutrophils, mast cells, monocytes, B cells, and NK cells are all made in the bone marrow.
    • T cells are made in the thymus, located above the heart.
    • B cells differentiate into plasma cells, which secrete antibodies during immune responses.
    • Mast cells reside in almost all tissues once produced and are involved in the inflammatory response.

    Primary Lymphoid Organs

    • Bone marrow is the site of blood cell production (hematopoiesis) for B cells and immature T cells.
    • B cells mature in the bone marrow.
    • Thymus, located above the heart, is the site where T cells mature.
    • The thymus contains T cells, dendritic cells, macrophages, and epithelial cells.
    • The thymus atrophies with age, resulting in a decrease in size and function.

    Secondary Lymphoid Organs

    • Lymph nodes are distributed throughout the body.
    • Lymph nodes collect lymphatic fluid from the open lymphatic system and act as filters for microbes.
    • Macrophages within the lymph nodes phagocytose microbes that enter via the lymph fluid.
    • Lymphoid nodules include tonsils, appendix, Peyer's patches, and mucosal-associated lymphoid tissues (MALT).
    • The spleen is the largest lymphoid organ.
    • The spleen removes microbes and old red blood cells (RBCs).

    Innate Immune System Barriers

    • Mechanical: Physical structures that block entry of foreign particles.

      • Skin: Continuous, water-resistant layer preventing entry.
      • Tight junctions between epithelial cells seal gaps, blocking transcellular pathways.
      • Hair and cilia: Move particles trapped in mucus towards digestive tract for destruction.

    • Chemical: Secretions that create hostile environments for invaders.

      • Mucus: Traps foreign particles for removal.
      • Enzymes: Lysozymes in tears break down bacterial cell walls.
      • Sebum: Oil makes skin water-resistant.
      • Gastric juices: Low pH (highly acidic) creates an environment that is unfavorable for invaders.

    • Microbiological: Competition with naturally occurring bacteria.

      • Symbiotic flora: The body's naturally present bacteria compete with new invaders.

    Humoral Factors

    • Inhibit pathogen growth and spread
    • Include inflammatory factors (interleukins, TNF) that cause fever
    • Interferons (Type 1) are proteins released to inhibit viral replication
    • Anti-microbial substances such as cytokines, complement proteins, and the complement system of plasma proteins
    • Complement proteins, like C3b, bind to pathogens for phagocytosis, a process known as opsonization
    • Fe-binding proteins, including transferrin, restrict iron availability to limit pathogen growth

    Cellular Factors

    • Phagocytic cells: neutrophils, macrophages, dendritic cells
    • Inflammatory mediator cells: basophils, eosinophils, mast cells
    • Natural Killer Cells

    Inflammatory Response

    • A non-specific response to tissue damage, involving a series of sequential events.

    • Three main stages: vasodilation, emigration of phagocytes, and tissue repair.

    Vasodilation

    • Increased blood flow to the damaged region.
    • Facilitates the delivery of immune cells and other necessary components to the site of injury.

    Emigration of Phagocytes

    • Entry of phagocytic cells (neutrophils and macrophages) into the damaged area.
    • This occurs through squeezing between the endothelial cells lining the blood vessels.
    • Phagocytes engulf and destroy pathogens, cellular debris, and foreign substances.

    Tissue Repair

    • The final stage of the inflammatory response.
    • Aim: restore damaged tissue to its original state.
    • Involves growth of new cells and the formation of new tissue.
    • The process may involve the production of scar tissue in severe cases.

    Major Histocompatibility Complexes (MHC)

    • All nucleated cells in the body express MHC molecules, which are membrane-bound proteins.
    • MHC molecules display antigens to immune system cells.
    • MHC Class I molecules are expressed by nucleated cells.
    • MHC Class I molecules display antigens derived from the cell’s own metabolism (a subset of cellular proteins).
    • Displaying these antigens protects the cell from destruction by the immune system.
    • HLA-A, HLA-B, and HLA-C are human leukocyte antigens, which are MHCs.

    MHC & Immune System Cells

    • Infections, cancer, or foreign agents create different antigens.
    • These antigens bind to CD8+ Natural Killer (NK) cells.
    • Binding of antigens triggers the release of cytokines by NK cells, leading to cell death.
    • NK cells also induce cell death in cells lacking MHC I.
    • NK cells are similar to cytotoxic T cells (Tc), but they do NOT bind specific antigens.
    • NK cells release granzyme and perforin.
    • Granzyme and perforin create pores in the cell membrane.
    • The creation of pores leads to an influx of water and ions, causing the cell to swell and burst.
    • This process is referred to as the "endogenous pathway".

    MHC Class II

    • MHC Class II molecules are expressed on antigen-presenting cells (APCs) such as B cells, dendritic cells, and macrophages.
    • APCs engulf antigens through phagocytosis.
    • These antigens are then processed and presented on the cell surface by MHC Class II molecules.
    • CD4+ helper T-cell receptors can recognize these antigen-MHC II complexes.
    • This recognition initiates the adaptive immune response.
    • In humans, MHC Class II molecules are called HLA-DP, HLA-DQ, and HLA-DR.
    • This antigen-processing and presentation pathway is called the "exogenous pathway".
    • MHC Class II presentation of antigens is a crucial step the immune system's ability to detect and respond to pathogens.

    Phagocytosis

    • Phagocytosis is a process where cells engulf and break down particles like microbes or old cells.
    • Phagocytosis involves four steps: adhering, ingestion, digestion, and killing.
    • The end products of digestion are released into or out of the cell.

    Types of Phagocytes

    • Macrophages are phagocytes that reside in tissues.
    • Neutrophils are phagocytes recruited to infection sites through chemotaxis.
    • Monocytes develop into macrophages and dendritic cells.

    Chemotaxis and Neutrophil Recruitment

    • Chemokines attract neutrophils in the blood plasma to infection sites.
    • Neutrophils undergo margination, adhering to the endothelial wall of blood vessels.
    • Neutrophils then undergo diapedesis, squeezing between endothelial cells to reach the infection site.
    • Neutrophils die during the process of killing bacteria, leading to the formation of NETs (neutrophil extracellular traps).
    • NETs are a mix of chromatin, granules, and cytoplasmic proteins from lysed neutrophils.
    • Pus is formed from a mixture of dead neutrophils and the infectious agent.

    Recognition of Microbes

    • Phagocytes recognize microbes by detecting PAMPs (pathogen-associated molecular patterns).
    • PAMPs are unique conserved structures found on the surface of microbial cells.
    • Examples of PAMPs include lipopolysaccharide (LPS) in gram-negative bacteria and peptidoglycan (PGN) in bacterial cell walls.
    • PRRs (pattern recognition receptors) are present on immune cells like macrophages and bind to PAMPs.
    • Toll-like Receptors (TLRs) are an example of PRRs, acting as transmembrane receptors with an extracellular domain that recognizes PAMPs and an intracellular signalling domain that is activated upon binding.

    Antigens

    • Antigens are molecules that trigger an immune response.
    • They are recognized by antibodies, which are proteins produced by the immune system.
    • Antigens can be whole cells, parts of cells, or single molecules.
    • Examples of antigens include:
      • Transplanted tissue grafts
      • Incompatible blood cells
      • Spike protein on COVID-19 virus
      • Pollen
      • Egg white protein
    • Antigens possess two key characteristics:
      • Reactivity: They bind specifically to antibodies.
      • Immunogenicity: They provoke an immune response, leading to antibody production.

    Antibodies

    • Antibodies are proteins that bind specifically to antigens.
    • They are produced by B cells, a type of white blood cell.
    • They play a crucial role in adaptive immunity, which is the body's ability to defend against specific pathogens.

    Adaptive Immunity

    • Adaptive immunity is the body's ability to recognize and defend against specific pathogens.
    • It involves the activation of lymphocytes, specifically B cells and T cells.
    • Adaptive immunity relies on the "memory" of previously encountered antigens.
    • The adaptive immune response involves three stages:
      • Antigen Recognition: Lymphocytes recognize and bind to specific antigens.
      • Lymphocyte Activation: Helper T cells release cytokines that activate both B and T cells.
      • Attack Launch: Activated B and T cells launch an attack against the antigen-bearing substance.
    • B cells (Humoral):
      • Recognize antigens.
      • Transform into plasma cells in bone marrow.
      • Synthesize and secrete antibodies.
      • Antibodies bind, precipitate, or trigger the destruction of antigen-bearing substances.
    • Memory B Cells:
      • Some B cells do not become plasma cells and instead develop into memory B cells.
      • Circulate in the body system indefinitely.
      • "Save" the antibody design for future infections.
      • Lead to a faster and stronger immune response upon re-infection.

    Basophils and Immune Response

    • Basophils are granulocytes that play a crucial role in allergic reactions and parasitic infections.
    • Upon activation, they release histamine, heparin, and other inflammatory mediators that contribute to the inflammatory response.

    Initial Non-Specific Response

    • Neutrophils are the first line of defense against pathogens, acting as phagocytes to engulf and destroy invading microbes.

    Inappropriate Immune Activation

    • Overactive or misdirected immune responses can lead to autoimmune disorders, hypersensitivities, or chronic inflammation, causing tissue damage and disease.

    Neutrophils and Immune Response

    • Neutrophils are short-lived, highly mobile phagocytic cells that engulf and destroy invading microbes, particularly bacteria.
    • They release enzymes and reactive oxygen species to kill pathogens.

    Macrophages and Dendritic Cells Development

    • Monocytes are phagocytic cells that differentiate into macrophages and dendritic cells in tissues.

    Eosinophils and Immune Response

    • Eosinophils are primarily involved in combating parasitic infections and allergic reactions.
    • They release cytotoxic substances to target and destroy parasites and contribute to the inflammatory response in allergies.

    Myeloid Stem Cells and Immune Cell Origin

    • Myeloid stem cells differentiate into various immune cells, including neutrophils, basophils, eosinophils, monocytes, macrophages, and dendritic cells.

    Mast Cells and Immune Response

    • Mast cells primarily mediate type I hypersensitivity reactions, releasing histamine and other inflammatory mediators that contribute to allergic responses.

    Monocyte Transformation

    • Monocytes differentiate into macrophages and dendritic cells, which are crucial antigen-presenting cells in the adaptive immune response.

    Mast Cell Activation Effects

    • Mast cell activation can cause unintended effects like anaphylaxis, a severe allergic reaction that can be life-threatening.

    B Cells and Adaptive Immunity

    • B cells are responsible for humoral immunity, producing antibodies that specifically target and neutralize pathogens.

    Cytotoxic T Cell Involvement

    • Cytotoxic T cells, also known as CD8+ T cells, directly destroy infected cells by releasing cytotoxic substances.

    Natural Killer (NK) Cells Function

    • NK cells are part of the innate immune system and target and destroy infected cells or cancerous cells without prior sensitization to specific antigens.

    Adaptive Immunity Response Following First Exposure

    • After the first exposure to an antigen, adaptive immunity remembers the antigen and mounts a quicker and more effective response upon subsequent encounters.

    Cytokines and T Cell Function

    • Cytokines are signaling molecules that regulate the activity of T cells, promoting their differentiation, activation, and proliferation.

    B Cell Differentiation

    • B cells differentiate into plasma cells that produce antibodies and memory B cells that persist in the body to provide long-term immunity.

    Mast Cell Function

    • Mast cells release histamine and other inflammatory mediators to enhance the inflammatory response, a critical part of the immune defense.

    Lymphocyte Generation Location

    • Lymphocytes, including B and T cells, are primarily generated in the bone marrow, the main site of hematopoiesis.

    Primary Site of Hematopoiesis

    • The bone marrow is the primary site of hematopoiesis, where all blood cells, including immune cells, are produced.

    Lymphoid Organ For B and NK Cell Activation

    • The spleen is a secondary lymphoid organ, primarily responsible for activating B cells and NK cells.

    Immune Response and B Cell Changes

    • During an immune response, B cells proliferate, differentiate into plasma cells, and produce antibodies specific for the encountered antigen.

    T Cell Development Location

    • T cells undergo maturation and selection in the thymus, a primary lymphoid organ where they develop the ability to recognize and respond to specific antigens.

    Mast Cell Function in Immune Response

    • Mast cells release histamine and other inflammatory mediators to initiate the inflammatory response, a critical part of innate immunity.

    B Cell Maturation Location

    • The bone marrow is the primary site where B cells mature, acquiring the ability to produce antibodies.

    Spleen Function in Lymphatic System

    • The spleen is a vital part of the lymphatic system, filtering blood, removing old or damaged red blood cells, and activating B and NK cells.

    Lymphatic Fluid Filtration

    • Lymph nodes are responsible for filtering lymphatic fluid, removing pathogens and cellular debris, and presenting antigens to immune cells.

    Thymus Changes with Age

    • The thymus gradually shrinks with age, affecting its ability to produce new T cells and contributing to a decline in immune function with aging.

    Thymus Components

    • The thymus contains T cells at different stages of maturation, epithelial cells that provide a microenvironment for T cell development, and macrophages that remove apoptotic T cells.

    Tight Junctions and Immune System

    • Tight junctions between epithelial cells create a barrier that prevents the entry of pathogens into the body, contributing to innate immunity.

    Hostile Environment in Digestive Tract

    • Gastric acid secreted in the stomach creates a hostile environment that kills many pathogens, providing an initial barrier against infection.

    Skin Contribution to Innate Immunity

    • The skin forms a physical barrier, preventing the entry of pathogens, and contains antimicrobial peptides and sebaceous glands that produce antimicrobial substances.

    Symbiotic Flora and Immune System

    • Commensal bacteria in the gut, known as symbiotic flora, compete with pathogens for resources and produce antimicrobial substances, contributing to the overall immune balance.

    Mucus in Innate Response

    • Mucus traps pathogens and prevents their adherence to tissues, contributing to the physical barrier of the innate immune response.

    Type 1 Interferons in Immune Response

    • Type 1 interferons inhibit viral replication and activate immune cells, playing a crucial role in the antiviral response.

    Tagging for Phagocytosis

    • Complement proteins, particularly C3b, tag pathogens for phagocytosis by binding to their surface, enhancing their recognition and destruction by phagocytes.

    Fe-Binding Proteins Function

    • Fe-binding proteins in serum and other fluids sequester iron needed by pathogens, making it unavailable for their growth and survival, inhibiting their infection.

    Phagocytes in Immune Response

    • Neutrophils, macrophages, and dendritic cells are all phagocytes that engulf and destroy pathogens, playing a central role in innate immunity.

    Functions of Inflammatory Mediator Cells

    • Inflammatory mediator cells release histamine, prostaglandins, and other molecules that promote inflammation, a crucial component of the immune response.

    Inflammatory Response Stages

    • First stage - Vascular changes: Vasodilation increases blood flow, causing redness and heat, while increased vascular permeability allows fluid and cells to leak into the damaged tissue.
    • Second stage - Cellular infiltration: Phagocytic cells like neutrophils and macrophages migrate to the site of inflammation to engulf and destroy pathogens.
    • Third stage - Tissue repair: The final stage involves the removal of debris, the growth of new tissue, and the restoration of function.

    Vasodilation Effect

    • Vasodilation increases blood flow to the injured area, delivering immune cells and nutrients necessary for repair.

    Tissue Repair Stage Function

    • The tissue repair stage aims to restore the damaged tissue to its original state, replacing damaged cells and rebuilding the functional organization.

    Inflammatory Response Stage Sequence

    • The inflammatory response follows a sequential pattern: Vascular changes, Cellular infiltration, and Tissue Repair.

    MHC Class I Molecules Role

    • MHC class I molecules present intracellular antigens, often viral or tumor proteins, to cytotoxic T cells, allowing them to recognize and destroy infected or abnormal cells.

    Natural Killer (NK) Cells Role

    • NK cells provide an important first line of defense against viruses and cancer by destroying cells that lack MHC class I expression or other markers of stress.

    Lack of MHC Class I Molecules

    • The absence of MHC class I molecules on a cell surface can signal infection or tumor formation, prompting NK cells to destroy it.

    NK Cell vs. Cytotoxic T Cells

    • NK cells differ from cytotoxic T cells in their mechanism of action: NK cells recognize and kill target cells based on the absence of MHC class I molecules or other stress markers, while cytotoxic T cells require specific antigen presentation by MHC class I molecules.

    Endogenous Pathway

    • The endogenous pathway involves the processing and presentation of intracellular antigens, often viral or tumor proteins, by MHC class I molecules to cytotoxic T cells.

    Antigen-Presenting Cells (APCs) Role

    • Antigen-presenting cells (APCs), like macrophages, dendritic cells, and B cells, capture and process antigens, presenting them to T cells to initiate an adaptive immune response.

    Exogenous Pathway

    • The exogenous pathway involves the uptake and processing of extracellular antigens, such as bacterial proteins, by APCs and their presentation on MHC class II molecules to helper T cells.

    HLA Molecules classified under MHC Class II

    • HLA-DP, HLA-DQ, and HLA-DR are classified under MHC class II, and they are critical players in the adaptive immune response.

    MHC Class II Molecule Users

    • Macrophages, dendritic cells, and B cells utilize MHC class II molecules for antigen presentation, a vital step in activating the adaptive immune response.

    MHC Class II and Immune Activation

    • The detection of antigens displayed on MHC class II molecules primarily activates helper T cells, triggering the adaptive immune response.

    Neutrophil Function in Killing Bacteria

    • Neutrophils release reactive oxygen species (ROS) and enzymes to kill infectious bacteria, forming neutrophil extracellular traps (NETs) to capture and neutralize pathogens.

    Chemokine Role in Immune Response

    • Chemokines are chemical messengers that guide the migration of immune cells to the site of inflammation.

    PAMPs and Immune System

    • Pathogen-associated molecular patterns (PAMPs) are molecular structures found on pathogens, recognized by immune cells through pattern recognition receptors (PRRs).

    Monocyte Function

    • Monocytes are phagocytic cells that differentiate into macrophages and dendritic cells, playing a crucial role in both innate and adaptive immunity.

    Macrophage Receptors

    • Macrophages express pattern recognition receptors (PRRs) like Toll-like receptors (TLRs) that recognize PAMPs on pathogens, triggering immune responses.

    Antigen Characteristics for Immune Response

    • Antigens must possess specific features, including foreignness, size, and chemical complexity, to trigger an immune response.

    Memory B Cell Role

    • Memory B cells persist in the immune system after an initial infection, allowing for a faster and more robust antibody response upon re-exposure to the same antigen.

    Adaptive Immune Response Activation

    • The activation of lymphocytes, particularly B and T cells, marks the initiation of the adaptive immune response.

    B Cell Fate after Antigen Recognition

    • Upon recognizing an antigen, B cells proliferate and differentiate into plasma cells that produce antibodies and memory B cells that provide long-term immunity.

    Antibody Effectiveness

    • Antibodies are effective against specific antigens due to their unique structure, with variable regions that bind to specific epitopes on the target antigen.

    T Cell Role in Immune Response

    • T cells are crucial players in cell-mediated immunity, recognizing and responding to infected or abnormal cells, and coordinating the immune response.

    Infected Cell Recognition by T Cells

    • Cytotoxic T cells (CD8+ T cells) identify infected body cells by recognizing antigen fragments presented on MHC class I molecules.

    Infected Cell Recognition Mechanism

    • Cytotoxic T cells recognize infected cells through the interaction of their T cell receptor (TCR) with a specific antigen presented by MHC class I molecules.

    MHC I and T Cell Activation

    • MHC I molecules display intracellular antigens to cytotoxic T cells, triggering their activation and subsequent destruction of infected cells.

    Antibody Significance for T Cells

    • The presence of antibodies on the membranes of T cells, particularly helper T cells, enables them to communicate with B cells and enhance antibody production.

    Dendritic Cells

    • Dendritic cells pick up pathogens and travel to lymph nodes, where they interact with B and T cells.

    Macrophages and Antigen Presentation

    • Macrophages engulf pathogens through phagocytosis.
    • Macrophages present processed antigens on MHC II receptors.

    Helper T Cell Activation

    • Helper T cells (TH) are activated by antigen presentation on MHC II.
    • Activated TH cells stimulate the production of more T and B cells.

    T Cell Activation

    • T cell receptors recognize specific antigen peptides presented on MHC I.
    • Co-stimulatory molecules CD28 on the T cell and B7 on the antigen-presenting cell (APC) are essential for T cell activation.
    • Cytokines, such as IL-1 and TNF, released by APCs contribute to T cell activation.

    Checkpoint Inhibition

    • Control of Immune System Activation: Mechanisms exist to regulate and prevent excessive immune activation.
    • "Shut Off" Mechanism: These mechanisms act as a "shut off" switch for the immune response, ensuring it does not become uncontrolled.
    • CTLA-4 and PD-1 Block T Cell Activation: CTLA-4 and PD-1 are molecules expressed on certain immune cells that act as checkpoints to inactivate T cells.
    • Displacement of CD28: CTLA-4 can displace CD28 from its binding site on antigen-presenting cells (APCs). This prevents T cell activation signals.
    • Upregulation of Checkpoint Molecules: Immune cells upregulate CTLA-4 or PD-1 during an immune response. This leads to inhibition of T cell activation and limits the immune reaction.

    T Cell Activation and Antibodies

    • Antibodies can be used to manipulate T cell activation, either interfering with or enhancing its function.
    • PD-1 is a protein on T cells that interacts with PD-L1, a protein expressed by many cells.
    • When PD-1 and PD-L1 bind, it leads to T cell inactivation and can even trigger cell death.
    • Using PD-1 antibodies to block the interaction between PD-1 and PD-L1 can prevent inactivation and keep T cells active.
    • CLA-4 antibodies are a different type of antibody that also targets T cell activation.
    • CLA-4 antibodies work by preventing T cell inactivation, effectively keeping T cells active indefinitely.

    Antibodies

    • Antibodies are immunoglobulins (Ig) that belong to a protein group called globulins.
    • Each antibody has four peptide chains: two heavy chains and two light chains.
    • Antigen-binding site is a unique site created in each B or T cell, responsible for recognizing and binding to specific antigens.
    • Variable Region is the antigen-binding fragment (FAB) or hypervariable region.
    • Constant Region provides the structural part of the Ig protein and is consistent between antibodies.
    • Fc region is the stem or crystallized fragment which determines the class of Ig an antibody belongs to.

    Immunoglobulin Classes

    • IgG: the most common antibody in the body, dominant in secondary immune responses (2nd exposure to antigen).
      • Passive Immunity: IgG can cross the placenta to provide passive immunity to the fetus and is found in breast milk.
      • Structure: IgG is a monomer (single molecule) with 4 polypeptide chains and 2 antigen binding sites.
    • IgA: Found in mucosal tissues like the digestive system and respiratory system (MALT - Mucosa-associated Lymphoid Tissue). Also found in breast milk, passing passive immunity to the infant with breast milk.
      • Structure: IgA is a dimer with 4 antigen binding sites.
    • IgM: The first antibody produced in the primary immune response; also produced in the secondary response.
      • Complement Activation: IgM is very effective at activating complement, a part of the immune system.
      • Structure: IgM is decavalent (10 binding sites), the largest antibody.
    • IgD: Primarily found in the prenatal stage.
    • IgE: Involved in allergic responses.

    Antibody Functions

    • Antibodies can neutralize antigens by preventing bacteria from releasing toxins.
    • Antibodies can agglutinate or precipitate antigens, making them easier for phagocytes to engulf.
    • Antibodies can activate the complement system, leading to cell lysis.
      • The binding of an antibody to an antigen triggers a cascade of events, culminating in the formation of a membrane attack complex (MAC).
      • The MAC is formed from complement proteins C5-C9 and inserts itself into the cell membrane, creating a pore that allows ions and fluid to enter the cell, causing it to swell and lyse.
    • Antibodies can opsonize pathogens, tagging them for elimination by phagocytes.
    • Antibodies can trigger antibody-dependent cellular cytotoxicity (ADCC), leading to the destruction of cells by cytotoxic T cells.
      • When specific antibodies bind to cell membranes, they can trigger cytotoxic T cells to release cytotoxic substances that lead to cell death.

    Active Immunity

    • Active immunity happens when the body's immune system responds to an infection. This response creates memory B cells that provide long-lasting protection.
    • There are two types: natural and artificial.
    • Natural active immunity: occurs when a person is naturally exposed to an infectious agent, like contracting Covid-19.
    • Artificial active immunity: occurs when a person is intentionally exposed to an infectious agent, such as through vaccination with a weakened or inactivated virus. Covid-19 vaccines use a single antigen protein that elicits an antibody response without containing the live virus.
    • Vaccines typically contain small quantities of a pathogen (inactive or weakened), toxins, or harmless antigenic molecules (like surface proteins) from the virus or microorganism.

    Passive Immunity

    • Direct transfer of antibodies from one person to another.
    • Provides temporary protection.
    • No memory B cells are generated.

    Types of Passive Immunity

    • Natural: Occurs through the transfer of IgG antibodies from the mother to the fetus via the placenta.
    • Natural: IgA antibodies are present in breast milk transferred to the infant.
    • Artificial: Occurs when serum containing antibodies from an infected or vaccinated person is administered.

    Active Immunity

    • B cells undergo positive clonal selection, which is activation by T cells that are bound to the same antigen.
    • This occurs within the spleen, lymphoid tissue, or lymph nodes in the presence of an infectious agent.
    • B cells then differentiate into two types:
      • Plasma B cells, which secrete antibodies to fight infection.
      • Memory B cells, which are maintained in the body to provide a faster response to infection if it occurs again in the future.

    Booster Vaccinations

    • Booster vaccinations re-initiate the activation of memory B cells by T helper cells (TH).
    • This increases the division and supply of memory B cells to respond faster to a specific infection.
    • Serum memory B cells for a specific infection can decline over time.

    Lymphocyte Development

    • Lymphocytes must develop antigen receptors through genetic recombination.
    • Recombination Activating Genes (RAGs) splice out unused segments of the DNA, retaining only some V/D/J segments.
    • This processes starts with a Pro-B cell that expresses RAGs and forms a Pre-B cell.
    • The Pre-B cell produces an antibody with a non-specific heavy/light chain that cannot bind an antigen.
    • Terminal deoxynucleotidyl transferase (TdT) inserts single nucleotides to introduce frame-shift mutations, further increasing diversity of sequence outcomes.
    • B-cells undergo class switching after maturation to swap the heavy chain and create a different antibody Ig protein with the same variable regions.

    Primary Exposure to Antigens

    • The primary response to a foreign pathogen or vaccine is the initial exposure to the antigen.
    • The immune system takes time to develop antibodies that are specifically effective against that particular threat.

    Secondary Exposure to Antigens

    • The secondary response occurs when the body is re-exposed to the same antigen.
    • This response is much faster due to the presence of memory cells, leading to a quicker production of antibodies.
    • Antibody production during the secondary response is significantly higher, resulting in a more robust immune response.
    • The increased antibody production helps to reduce the severity of infection.
    • In many cases, individuals may not even notice they are infected due to the effectiveness of the secondary immune response.

    Self-Antigen Testing

    • The body usually does not produce antibodies against its own antigens (self-antigens) to prevent autoimmune responses.
    • MHCs (major histocompatibility complexes) are unique to each person except for identical twins.
    • Immune tolerance, the body's ability to distinguish between self and non-self, develops during fetal and postnatal life.
    • This tolerance is achieved through clonal inactivation and apoptosis (programmed cell death) of immune cells that produce antibodies matching the body's antigens.
    • During fetal development and in the thymus gland, approximately 95% of T cells are destroyed through a selection process.
    • T cells are required to recognize MHC class II molecules; failure to do so results in their destruction (negative selection).
    • T cells must NOT recognize MHC class I molecules (self-antigens); recognition leads to their destruction (negative selection).

    Cytotoxic T-cell Killing of Virus-Infected Cells

    • Virus-infected cells display altered metabolism, leading to the presentation of viral antigens on the cell surface via MHC I receptors.
    • Cytotoxic T cells (CTLs), which express CD8+ receptors, recognize and bind to these MHC I-antigen complexes.
    • Binding triggers the release of cytotoxic molecules by the CTL:
      • Perforin: a pore-forming protein that creates holes in the infected cell's membrane.
      • Granzymes: enzymes that enter the cell through the pores and trigger programmed cell death (apoptosis).
    • The influx of fluid and ions through the perforin pores ultimately causes the infected cell to burst and die.

    Altered Infection Resistance (Immunosuppression)

    • Protein or calorie malnutrition is the primary cause of immunosuppression globally, hindering the body's ability to produce antibodies (immunoglobulins) and maintain immune function.
    • Pre-existing diseases can lead to impaired immune responses, making the body more susceptible to subsequent infections.
    • Stress negatively impacts the immune system, increasing vulnerability to both cancer and infections.
    • Sleep deprivation has been directly linked to decreased immune system function.
    • Regular exercise and fitness can improve overall immune function and enhance the body’s resistance to infections.

    Immune Responses to Tissue Grafts and Organ Transplants

    • MHC-I on graft cells and MHC II on macrophages in transplant tissue can be different from the recipient's, triggering the recipient's immune T cells to attack and destroy the graft.

    • Rejection of grafts is a common immune response due to the differences in MHC proteins.

    • Medications like cyclosporine can help prevent rejection by blocking cytokine production from T cells, which eliminates the necessary proliferation signals for both TH and Tc cells.

    • Drugs that kill actively dividing lymphocytes (activated T cells) are used to decrease the recipient's T cell population, suppressing the immune response.

    Transfusion Reactions

    • Red blood cells (RBCs) lack a nucleus and MHC I, but have plasma membrane proteins (Rh factor) and carbohydrates (ABO system) that act as antigens.
    • Rh antibodies can be produced by an Rh-negative mother who has carried an Rh-positive fetus during a previous pregnancy, miscarriage, ectopic pregnancy, or blood transfusion.
    • In subsequent pregnancies with an Rh-positive fetus, these antibodies can cross into the fetal bloodstream and attack their blood cells.
    • Prevention of this immune response involves giving the mother anti-Rh antibodies to destroy any produced by her system after exposure from the first pregnancy.

    Allergic Reactions

    • Allergic reactions occur when the body overreacts to a substance that most people tolerate well.
    • Reactions can be immediate hypersensitivity (occurring immediately after exposure), or delayed hypersensitivity (between 12-72 hours after exposure).
    • During an allergic reaction, the allergen is recognized by B cells that differentiate into plasma cells, producing IgE and memory B cells.
    • These memory B cells allow for a quicker response to future exposures.
    • IgE, in the presence of an allergen, activates mast cells, triggering an allergic response, usually localized to the site of the antigen.

    Excessive Inflammatory Response (Anaphylaxis)

    • A severe allergic reaction caused by the release of histamine from mast cells or basophils into the bloodstream.
    • Leads to systemic effects, including severe hypertension (high blood pressure) and bronchial constriction (narrowing of the airways)
    • Can be life-threatening due to circulatory and respiratory failure.
    • Triggered by allergens such as bee stings, peanuts, medications, latex, and others.

    Autoimmune Disease

    • Occurs when the immune system mistakenly attacks the body's own proteins, treating them as foreign antigens.
    • Examples include Type 1 diabetes, multiple sclerosis, rheumatoid arthritis, myasthenia gravis, and lupus.

    Primary Lymphoid Organs

    • Responsible for production and maturation of lymphocytes
    • Bone marrow: Site of B cell maturation
    • Thymus: Site of T cell maturation

    Secondary Lymphoid Organs

    • Responsible for activation of lymphocytes in response to antigens
    • Examples include:
      • Lymph nodes
      • Lymphoid nodules (tonsils, appendix, MALT)
      • Spleen

    Humoral Factors

    • Humoral factors are molecules that discourage the growth and spread of pathogens.
    • Examples include:
      • Inflammation mediators: interleukins and TNF
      • Interferons: proteins released to inhibit viral replication (specifically Type 1 interferons)
      • Antimicrobial substances: cytokines, chemokines, and the complement system
        • Complement system consists of plasma proteins like C3b that can bind to and "tag" pathogens for phagocytosis, a process known as opsonization.
      • Iron-binding proteins: such as transferrin, which restrict the availability of iron, limiting pathogen growth.

    Cellular Factors

    • Phagocytic cells are cells that engulf and destroy pathogens.

      • Neutrophils: the most abundant white blood cells, are first responders to infection.
      • Macrophages: large phagocytic cells that reside in tissues and engulf pathogens.
      • Dendritic cells: antigen-presenting cells (APCs) that capture pathogens and present them to T cells.

    • Inflammatory mediator cells: release substances that contribute to inflammation.

      • Basophils: release histamine and other inflammatory mediators.
      • Eosinophils: important in the defense against parasites.
      • Mast cells: reside in tissues and release histamine and other inflammatory mediators, playing a role in allergic responses.

    • Natural killer (NK) cells: lymphocytes that kill infected cells and cancer cells without needing prior exposure to a specific antigen.

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    Explore the key components and functions of the immune system, including the innate immune response. Learn about granulocytes and agranulocytes, their roles in defending the body against pathogens and abnormal cells. This quiz will test your knowledge on how the immune system operates and its significance in health.

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