Immune Response Types and Cells
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Questions and Answers

What are the primary components of innate immunity?

  • Interferons and antibodies
  • Epithelial barriers, phagocytic cells, and NK cells (correct)
  • T lymphocytes and B lymphocytes
  • Antibodies and cytokines

    • Which type of immunity is responsible for defense against intracellular microbes?

  • Humoral immunity
  • Passive immunity
  • Cell-mediated immunity (correct)
  • Natural immunity

    • What is the CD4:CD8 ratio in T-lymphocytes?

  • 3:1
  • 4:1
  • 1:1
  • 2:1 (correct)

    • Which T-helper cell subset produces IL-4, IL-5, and IL-13?

    <p>TH2 subset</p> Signup and view all the answers

    What do CD4 molecules bind to?

    <p>Class II MHC molecules</p> Signup and view all the answers

    What is the main function of phagocytic cells in innate immunity?

    <p>Engulf and destroy microbes</p> Signup and view all the answers

    What is the role of the CD3 complex in T-lymphocytes?

    <p>Signal transduction</p> Signup and view all the answers

    What type of immunity is characterized by its ability to recognize nonmicrobial substances?

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

    Which cells mediate humoral immunity?

    <p>B lymphocytes</p> Signup and view all the answers

    Which cytokine is synthesized and secreted by TH1 cells?

    <p>IL-2</p> Signup and view all the answers

    What percentage of peripheral blood lymphocytes do natural killer cells (NK cells) make up?

    <p>10-15%</p> Signup and view all the answers

    Which of the following is a receptor used to identify NK cells?

    <p>CD16</p> Signup and view all the answers

    What is the primary role of natural killer cells?

    <p>To kill tumor cells and virally-infected cells</p> Signup and view all the answers

    Class I MHC molecules are present on which of the following?

    <p>All nucleated cells and platelets</p> Signup and view all the answers

    What type of antigens do class II MHC molecules present?

    <p>Exogenous antigens</p> Signup and view all the answers

    Which of the following statements about CD8+ T cells is true?

    <p>They are class I MHC-restricted.</p> Signup and view all the answers

    The genes encoding MHC molecules are clustered on which chromosome segment?

    <p>Chromosome 6</p> Signup and view all the answers

    NK cells secrete which cytokine to aid in immune response?

    <p>IFN-γ</p> Signup and view all the answers

    What type of maturation do CD4+ T cells undergo with class II MHC?

    <p>They do not mature without interaction with MHC.</p> Signup and view all the answers

    Which categories are MHC gene products classified into?

    <p>Class I, Class II, Class III</p> Signup and view all the answers

    What is the primary function of CD8+ T cells?

    <p>Kill other cells</p> Signup and view all the answers

    What is required for complete activation of T cells?

    <p>Engagement of TCR and interaction of CD28 with CD80/CD86</p> Signup and view all the answers

    Which immunoglobulin constitutes 95% of plasma immunoglobulins after B-cell activation?

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

    Which role is NOT associated with macrophages in the immune response?

    <p>Synthesize immunoglobulins</p> Signup and view all the answers

    Where are B-lymphocytes primarily found in the spleen?

    <p>White pulp</p> Signup and view all the answers

    Which immunoglobulin is primarily found as a monomer on the surface of B-cells?

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

    What do dendritic cells and Langerhan's cells have in common?

    <p>They contain large amounts of class II MHC molecules</p> Signup and view all the answers

    What happens to T cells in the absence of the second signal for activation?

    <p>They undergo apoptosis or become anergic</p> Signup and view all the answers

    What role do follicular dendritic cells serve in the germinal centers?

    <p>They contain antibodies on their cell surfaces</p> Signup and view all the answers

    What is the function of CD21 on B cells?

    <p>Serves as a complement receptor</p> Signup and view all the answers

    What characterizes the immediate phase of a Type I hypersensitivity reaction?

    <p>Vasodilatation and exudation</p> Signup and view all the answers

    What is a common consequence of a Type II hypersensitivity reaction?

    <p>Destruction through opsonization</p> Signup and view all the answers

    In which hypersensitivity reaction is cell-mediated immunity primarily involved?

    <p>Type IV hypersensitivity</p> Signup and view all the answers

    What type of hypersensitivity is characterized by the deposition of immune complexes in tissues?

    <p>Type III</p> Signup and view all the answers

    Which of the following is an example of Type II hypersensitivity?

    <p>Myasthenia gravis</p> Signup and view all the answers

    Which cells are primarily involved in the delayed-type hypersensitivity of Type IV hypersensitivity reactions?

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

    What mechanism is associated with antibody-dependent cellular cytotoxicity (ADCC)?

    <p>Target cell lysis by cytotoxic cells</p> Signup and view all the answers

    What time frame characterizes the late phase of a Type I hypersensitivity reaction?

    <p>2-4 hours after exposure</p> Signup and view all the answers

    Which of the following conditions is associated with Type III hypersensitivity?

    <p>Serum sickness</p> Signup and view all the answers

    Which statement regarding Type I hypersensitivity is correct?

    <p>It results in immediate symptoms.</p> Signup and view all the answers

    What characterizes the late phase of a Type I hypersensitivity reaction?

    <p>Inflammation due to leukocyte influx</p> Signup and view all the answers

    Which of the following is a mechanism of Type II hypersensitivity reactions?

    <p>Opsonization and cell-mediated phagocytosis</p> Signup and view all the answers

    What type of hypersensitivity reaction is mediated by antigen-activated T lymphocytes?

    <p>Type IV</p> Signup and view all the answers

    Which condition is commonly associated with Type III hypersensitivity reactions?

    <p>Systemic lupus erythematosus</p> Signup and view all the answers

    Which of the following best describes the role of antibody-mediated cellular dysfunction in Type II hypersensitivity?

    <p>Disrupting normal cellular functions</p> Signup and view all the answers

    How do antigen-antibody complexes primarily cause damage in Type III hypersensitivity?

    <p>By eliciting inflammation at deposition sites</p> Signup and view all the answers

    What duration characterizes the immediate phase of a Type I hypersensitivity reaction?

    <p>5-30 minutes</p> Signup and view all the answers

    What is a common example of an autoimmune condition associated with Type II hypersensitivity?

    <p>Autoimmune hemolytic anemia</p> Signup and view all the answers

    What main role do T-lymphocytes serve in the adaptive immune response?

    <p>Regulate the immune response and perform cellular defense</p> Signup and view all the answers

    What is the primary function of T-helper-2 (TH2) cells?

    <p>Produce cytokines like IL-4, IL-5, and IL-13</p> Signup and view all the answers

    Which of the following describes the CD4+ T cells' interaction with MHC molecules?

    <p>Bind to class II MHC molecules on antigen-presenting cells</p> Signup and view all the answers

    How does somatic rearrangement affect TCR diversity?

    <p>It enables the creation of diverse antigen receptors</p> Signup and view all the answers

    What characterizes innate immunity?

    <p>It consists of defense mechanisms present before infection</p> Signup and view all the answers

    What is a major characteristic of B lymphocytes in humoral immunity?

    <p>They secrete antibodies into circulation</p> Signup and view all the answers

    Which statement about T-cell receptors (TCR) is accurate?

    <p>TCRs consist of α and β chains connected by disulfide bonds</p> Signup and view all the answers

    What defines the adaptive immune response's ability to recognize antigens?

    <p>It can specifically adapt to both microbial and nonmicrobial substances</p> Signup and view all the answers

    What is the primary identifying marker for natural killer (NK) cells?

    <p>CD56</p> Signup and view all the answers

    How do NK cells recognize target cells?

    <p>Using a combination of activator and inhibitor receptors</p> Signup and view all the answers

    What type of antigens do class I MHC molecules typically present?

    <p>Intracellular viral proteins</p> Signup and view all the answers

    Which of the following molecules do CD4+ T helper cells recognize?

    <p>Class II MHC proteins</p> Signup and view all the answers

    Which part of the immune system does CD16 belong to?

    <p>NK cell activation</p> Signup and view all the answers

    Which class of MHC molecules is expressed on all nucleated cells?

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

    What mechanisms are implicated in hypersensitivity reactions?

    <p>T cell responses and antibody responses</p> Signup and view all the answers

    What is one function of the Main Histocompatibility Complex (MHC)?

    <p>Present peptide fragments to T cells</p> Signup and view all the answers

    What is the primary role of CD8+ T cells?

    <p>To kill infected or cancerous cells</p> Signup and view all the answers

    Which molecules do B cells express that are critical for their function?

    <p>CD40 and CD19</p> Signup and view all the answers

    How do T cells undergo complete activation?

    <p>By binding to specific MHC-antigen complexes with coreceptors and receiving additional signals</p> Signup and view all the answers

    Which immunoglobulin is mainly found in traces in the serum?

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

    What is a key characteristic of dendritic cells?

    <p>They express a large amount of class II MHC molecules</p> Signup and view all the answers

    What happens to T cells that do not receive a second signal during activation?

    <p>They undergo apoptosis or become unreactive</p> Signup and view all the answers

    Which function is NOT associated with macrophages?

    <p>Producing antibodies</p> Signup and view all the answers

    Where do B-lymphocytes primarily reside in the lymph nodes?

    <p>In the superficial cortex</p> Signup and view all the answers

    Which statement accurately describes innate immunity?

    <p>Innate immunity is present before infection occurs.</p> Signup and view all the answers

    What is the primary function of T-lymphocytes?

    <p>To recognize and eliminate infected or abnormal cells.</p> Signup and view all the answers

    Which T-cell subset predominantly binds to class II MHC molecules?

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

    Which cytokine is NOT produced by TH1 cells?

    <p>IL-4</p> Signup and view all the answers

    Which cell type is primarily responsible for humoral immunity?

    <p>B-lymphocytes</p> Signup and view all the answers

    What type of immunity arises from prior exposure to microbes?

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

    What role does somatic rearrangement play in T-cell receptors?

    <p>It generates diversity among TCRs.</p> Signup and view all the answers

    Which of the following describes the CD4:CD8 ratio in T-cells?

    <p>2:1</p> Signup and view all the answers

    What is the initial time frame for the immediate phase of a Type I hypersensitivity reaction?

    <p>5-30 minutes</p> Signup and view all the answers

    Which mechanism is NOT involved in Type II hypersensitivity reactions?

    <p>Sensitized T lymphocyte activation</p> Signup and view all the answers

    What type of immune response is primarily associated with Type IV hypersensitivity?

    <p>Cell-mediated immunity</p> Signup and view all the answers

    Which condition is an example of a Type III hypersensitivity reaction?

    <p>Systemic lupus erythematosus</p> Signup and view all the answers

    What characterizes the late phase of a Type I hypersensitivity reaction?

    <p>Influx of leukocytes and lasts for days</p> Signup and view all the answers

    Which of the following describes the mechanism of tissue damage in Type III hypersensitivity?

    <p>Inflammation at sites of immune complex deposition</p> Signup and view all the answers

    What type of antibodies are primarily involved in Type II hypersensitivity reactions?

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

    In Type IV hypersensitivity, sensitized T cells mainly respond to which type of antigens?

    <p>Intracellular pathogens</p> Signup and view all the answers

    What is the main identifying marker for natural killer cells?

    <p>CD16</p> Signup and view all the answers

    Which class of MHC molecules presents endogenous antigens?

    <p>Class I</p> Signup and view all the answers

    Which statement best describes the activator and inhibitor receptors of NK cells?

    <p>Activator receptors help identify potentially harmful cells; inhibitor receptors ensure normal cells remain unharmed.</p> Signup and view all the answers

    What is the primary function of class II MHC molecules?

    <p>To present exogenous antigens to CD4+ T cells.</p> Signup and view all the answers

    Which cytokine do natural killer cells primarily secrete?

    <p>IFN-γ</p> Signup and view all the answers

    What primary role does CD8 perform in T cell activation?

    <p>Serves as a co-receptor for MHC class I interactions.</p> Signup and view all the answers

    Which of the following genes encode components of the complement system?

    <p>Class III genes</p> Signup and view all the answers

    Where are class I MHC molecules commonly expressed?

    <p>On all nucleated cells and platelets</p> Signup and view all the answers

    What are the primary functions of CD8+ T cells?

    <p>Killing other cells and secreting TH1 cytokines</p> Signup and view all the answers

    What is the role of B-cells after antigen stimulation?

    <p>Transform into plasma cells that secrete immunoglobulins</p> Signup and view all the answers

    What is required for the complete activation of T cells?

    <p>Engagement of TCR by MHC-antigen complex with CD4/CD8 and interaction with CD28</p> Signup and view all the answers

    Which statement accurately describes roles of macrophages?

    <p>Participate in antigen presentation and the production of cytokines</p> Signup and view all the answers

    What molecule does CD40 on B-cells interact with?

    <p>CD154 on activated T-lymphocytes</p> Signup and view all the answers

    Where are B-lymphocytes primarily located in lymph nodes?

    <p>In the superficial cortex</p> Signup and view all the answers

    Which differences exist between dendritic cells and Langerhan's cells?

    <p>Dendritic cells are located in lymphoid tissue, while Langerhan's cells are in the epidermis</p> Signup and view all the answers

    What is the consequence for T cells if they do not receive the second signal for activation?

    <p>They undergo apoptosis or become anergic</p> Signup and view all the answers

    Study Notes

    Immune Response Types

    • Innate Immunity: Pre-existing defenses that recognize microbes and protect against infections.
      • Components:
        • Epithelial barriers, e.g., skin
        • Phagocytic cells: neutrophils, macrophages
        • Natural killer (NK) cells
        • Complement system proteins
    • Adaptive (Acquired) Immunity: Activated by microbes and can recognize non-microbial substances (antigens).
      • Types:
        • Cell-Mediated Immunity: Targets intracellular microbes; mediated by T lymphocytes (thymus-derived)
        • Humoral Immunity: Targets extracellular microbes and their toxins; mediated by B lymphocytes (bone marrow-derived) and secreted antibodies.

    Immune Cells

    • T Lymphocytes (T Cells): 60-70% of circulating lymphocytes.
      • T Cell Receptor (TCR): Recognizes specific antigens; composed of α and β polypeptide chains
        • Variable region: Binds to antigen
        • Constant region: Links to CD3 complex
      • CD3 Complex: Involves in signal transduction for TCR activation.
      • Somatic Rearrangement: TCR gene rearrangements create diversity.
      • CD4 and CD8 Coreceptors:
        • CD4: Present in about 60% of T cells; binds to class II MHC molecules on antigen-presenting cells.
        • CD8: Present in about 30% of T cells; binds to class I MHC molecules.
      • T Helper (TH) Cells:
        • TH1: Secrete IL-2 and IFN-γ; involved in delayed hypersensitivity, macrophage activation.
        • TH2: Secrete IL-4, IL-5, and IL-13; aid in antibody production and eosinophil activation.
      • CD8+ Cytotoxic T Cells: Kill other cells, primarily in cell-mediated immunity.
      • T Cell Activation: Requires two signals:
        • Engagement of TCR by appropriate MHC-antigen complex with CD4 or CD8 coreceptors.
        • Interaction of CD28 on T cells with CD80 or CD86 on antigen-presenting cells.
        • Absence of the second signal: T cells undergo apoptosis or become unresponsive (anergic), preventing autoimmune reactions.
    • B Lymphocytes (B Cells): 10-20% of circulating lymphocytes.
      • B Cell Receptor (BCR): Monomeric IgM; serves as an antigen receptor on B cells.
      • Somatic Rearrangement: Immunoglobulin gene rearrangements result in unique antigen specificity.
      • Other B Cell Molecules: CD19, CD20, CD21 (complement receptor, EBV binding), CD40 (interacts with CD154 on activated T cells)
      • Differentiation:
        • Plasma Cells: Secrete antibodies (IgG, IgM, IgA, IgE).
    • Macrophages: Role in immune response:
      • Antigen presentation to T cells via class II MHC molecules.
      • Cytokine production to influence other immune cells (T cells, B cells, endothelial cells, fibroblasts).
      • Secretion of toxic metabolites and enzymes to kill tumor cells.
      • Effector cells in delayed hypersensitivity reactions.
    • Dendritic Cells and Langerhan's Cells: Dendritic cell processes and high expression of class II MHC molecules.
      • Dendritic Cells: Found in lymphoid tissue.
      • Langerhan's Cells: Found in the epidermis.
      • Efficient Antigen Presentation: Low phagocytic activity.
      • Follicular Dendritic Cells: Found in germinal centers; express antibodies on their surface and have Fc receptors.
    • Natural Killer (NK) Cells: 10-15% of peripheral blood lymphocytes; not express TCR or immunoglobulin.
      • Larger Than Small Lymphocytes: Contains azurophilic granules (large granular lymphocytes).
      • Part of Innate Immunity: Kill tumor and virally infected cells without prior sensitization.
      • Identification: CD16 (Fc receptor for IgG-coated cells) and CD56.
      • Receptors:
        • Activators: Recognize ill-defined molecules on abnormal cells.
        • Inhibitors: Recognize class I MHC on normal cells.
      • Secrete IFN-γ:
    • Antibody-Dependent Cell Mediated Cytotoxicity (ADCC): NK cells kill antibody-coated cells via CD16.

    Major Histocompatibility Complex (MHC)

    • Function: Present peptide fragments of foreign proteins to T cells.
    • MHC Gene Location: Clustered on chromosome 6 (Human Leukocyte Antigen - HLA).
    • MHC Molecules:
      • Class I:
        • Expressed on all nucleated cells and platelets.
        • Encoded by HLA-A, HLA-B, and HLA-C loci.
        • Present endogenous antigens (e.g., viral proteins) to CD8+ cytotoxic T cells.
      • Class II:
        • Encoded by HLA-DP, HLA-DQ, and HLA-DR regions.
        • Present exogenous antigens (e.g., extracellular microbes) to CD4+ helper T cells.
      • Class III: Encode components of the complement system.

    Hypersensitivity Reactions (Types)

    • Type I (Immediate) Hypersensitivity: Rapid immune reaction within minutes of antigen-antibody binding to mast cells in previously sensitized individuals.
      • Phases:
        • Immediate Phase: Vasodilatation and exudation (5-30 minutes, subsiding in 60 minutes).
        • Late Phase: Leukocyte influx (2-4 hours, lasting for days)
      • Examples: Anaphylaxis, bronchial asthma, allergic rhinitis.
    • Type II Hypersensitivity: Antibodies against antigens on cell surfaces or extracellular matrix.
      • Mechanisms of Destruction:
        • Opsonization and Fc receptor-mediated phagocytosis.
        • Antibody-dependent cellular cytotoxicity (ADCC).
        • Antibody-mediated cellular dysfunction.
      • Examples:
        • Transfusion reactions
        • Autoimmune hemolytic anemia
        • Drug reactions
        • Myasthenia gravis
        • Grave's disease
        • Glomerulonephritis
        • Organ rejection
    • Type III Hypersensitivity: Immune complex (antigen-antibody) deposition in tissues causing inflammation and damage.
      • Examples:
        • Systemic lupus erythematosus
        • Polyarteritis nodosa
        • Poststreptococcal glomerulonephritis
        • Serum sickness.
    • Type IV (Cell-Mediated) Hypersensitivity: Initiated by antigen-activated T lymphocytes.
      • Mechanisms:
        • Delayed type hypersensitivity (CD4+ T cells): Defense against intracellular pathogens (e.g., mycobacteria, fungi, parasites).
        • Direct cell cytotoxicity (CD8+ T cells): Graft rejection.
      • Examples:
        • Contact dermatitis
        • Tuberculin reaction
        • Transplant rejection
        • Tumor immunity.

    The Immune Response

    • Innate immunity comprises defenses present before infection. It includes epithelial barriers, phagocytic cells, NK cells, and complement system proteins.
    • Adaptive immunity is stimulated by infection and can recognize microbes and nonmicrobial substances (antigens).
    • Cell-mediated immunity protects against intracellular microbes and is mediated by T lymphocytes.
    • Humoral immunity protects against extracellular microbes and toxins and is mediated by B lymphocytes and antibodies.

    Cells of the Immune System

    • T-lymphocytes

      • 60-70% of circulating lymphocytes.
      • Found in lymph nodes and spleen.
      • Contain a T cell receptor (TCR) composed of α and β chains with variable and constant regions.
      • Express CD4 or CD8 as coreceptors.
      • CD4+ T cells bind to class II MHC molecules and include TH1 (IL-2 and IFN-γ) and TH2 (IL-4, IL-5, and IL-13) subsets.
      • CD8+ T cells bind to class I MHC molecules and mainly function as cytotoxic cells.
      • T cell activation requires TCR engagement with MHC-antigen complex and CD28 interaction with CD80/CD86.

    • B-lymphocytes

      • 10-20% of circulating lymphocytes.
      • Found in lymph nodes and spleen.
      • Transform into plasma cells secreting antibodies.
      • Express a B cell receptor (BCR) composed of monomeric IgM.
      • Express CD19, CD20, CD21, and CD40.

    • Macrophages

      • Present antigens to T cells via class II MHC molecules.
      • Produce cytokines influencing other immune cells.
      • Secrete toxic metabolites and enzymes.
      • Important effector cells in delayed hypersensitivity.

    • Dendritic and Langerhans' cells

      • Have dendritic processes and high class II MHC expression.
      • Found in lymphoid tissue (dendritic) and epidermis (Langerhans).
      • Efficient antigen presenters with poor phagocytic activity.
      • Follicular dendritic cells in germinal centers trap antibodies.

    • Natural killer cells (NK cells)

      • 10-15% of peripheral blood lymphocytes.
      • Lack TCR and immunoglobulin.
      • Larger than lymphocytes and contain azurophilic granules.
      • Kill tumor and virally infected cells without prior sensitization.
      • Express CD16 and CD56.
      • Possess activating receptors and inhibitory receptors that recognize class I MHC.

    Major Histocompatibility Complex (MHC)

    • MHC molecules bind peptide fragments of foreign proteins for presentation to T cells.

    • Located on chromosome 6 and classified into class I, class II, and class III.

    • Class I MHC

      • Expressed on all nucleated cells and platelets.
      • Bind peptides derived from intracellular proteins (e.g., viral antigens).
      • Recognized by CD8+ cytotoxic T cells (class I MHC-restricted).

    • Class II MHC

      • Encoded by HLA-DP, HLA-DQ, and HLA-DR.
      • Present exogenous antigens (e.g., extracellular microbes).
      • Recognized by CD4+ helper T cells (class II MHC-restricted).

    Hypersensitivity Reactions

    • Type I (immediate) hypersensitivity: Rapid reaction (minutes) triggered by antigen-antibody binding to mast cells.

      • Immediate phase (5-30 min): Vasodilatation and exudation due to vasoactive amine release.
      • Late phase (2-4 hours): Influx of leukocytes (bronchial asthma, allergic rhinitis).

    • Type II hypersensitivity: Antibody-mediated against cell surface or extracellular matrix antigens.

      • Mechanisms: Opsonization and phagocytosis, antibody-dependent cell cytotoxicity (ADCC), antibody-mediated cellular dysfunction.
      • Examples: Transfusion reactions, autoimmune hemolytic anemia, drug reactions, myasthenia gravis, Grave's disease, glomerulonephritis, organ rejection.

    • Type III hypersensitivity: Immune complex-mediated.

      • Complexes deposit in various organs, primarily blood vessels.
      • Examples: Systemic lupus erythematosus, polyarteritis nodosa, poststreptococcal glomerulonephritis, serum sickness.

    • Type IV hypersensitivity (cell-mediated): Initiated by antigen-activated T lymphocytes.

      • Delayed type hypersensitivity: Mediated by CD4+ T cells, involved in defense against intracellular pathogens (mycobacteria, fungi, parasites), transplant rejection, tumor immunity.
      • Direct cell cytotoxicity: Mediated by CD8+ T cells, mainly encountered in graft rejection.

    Immune Response

    • Two major types of immune response: innate and adaptive
    • Innate immunity: present at birth, non-specific defense against pathogens
      • Components: epithelial barriers, phagocytes (neutrophils and macrophages), natural killer (NK) cells, complement system proteins
    • Adaptive immunity: develops after exposure to pathogens, highly specific
      • Two types: cell-mediated and humoral immunity

    Cells of the Immune System

    • T-lymphocytes (T cells): key players in cell-mediated immunity, originate in bone marrow and mature in thymus
      • T cell receptor (TCR): recognizes specific antigens
      • CD4 and CD8: coreceptors on T cells, determine which MHC class they interact with
      • CD4+ T cells: helper T cells, promote immune response by secreting cytokines
      • CD8+ T cells: cytotoxic T cells, directly kill infected or cancerous cells
    • B-lymphocytes (B cells): key players in humoral immunity, originate in bone marrow
      • Antibodies (immunoglobulins): secreted proteins that bind to antigens, neutralizing or eliminating pathogens
      • B cell receptor (BCR): recognizes specific antigens
      • Plasma cells: mature B cells that secrete antibodies
    • Macrophages: phagocytose foreign material and pathogens, present antigens to T cells
      • Produce cytokines: proteins that influence the behavior of other immune cells
      • Secrete toxic metabolites and enzymes: for killing tumor cells
    • Dendritic cells: antigen-presenting cells with dendritic processes
      • Efficient antigen presentation: activate T cells
      • Found in various tissues: lymph nodes, skin (Langerhan's cells)
    • Natural killer (NK) cells: part of innate immunity, kill infected or cancerous cells without prior sensitization
      • Don't express TCR or immunoglobulins: use other mechanisms for target recognition
      • Secrete IFN-γ: immune-stimulating cytokine

    Major Histocompatibility Complex (MHC)

    • MHC proteins: expressed on cell surfaces, present antigen fragments to T cells
    • Class I MHC: expressed on all nucleated cells, presents antigens from inside the cell (e.g., viral proteins)
      • Recognized by CD8+ T cells
    • Class II MHC: expressed on antigen-presenting cells (macrophages, dendritic cells), presents antigens from outside the cell (e.g., bacterial proteins)
      • Recognized by CD4+ T cells

    Hypersensitivity Reactions

    • Hypersensitivity: exaggerated or inappropriate immune response to an antigen, causing tissue damage
    • Type I (Immediate): mediated by IgE antibodies binding to mast cells, triggering immediate release of histamine and other inflammatory mediators
      • Examples: allergies, anaphylaxis
    • Type II (Cytotoxic): mediated by antibodies directed against cell surface antigens, leading to cell lysis or dysfunction
      • Examples: transfusion reactions, autoimmune hemolytic anemia
    • Type III (Immune Complex-mediated): mediated by antigen-antibody complexes depositing in tissues, triggering inflammation and tissue damage
      • Examples: systemic lupus erythematosus, serum sickness
    • Type IV (Cell-mediated): mediated by T cell activation, leading to delayed tissue damage
      • Examples: contact dermatitis, graft rejection, tuberculin skin test

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