Humoral and Cell-Mediated Immunity Notes PDF

**The Immune System: Humoral and Cell-Mediated Immunity** The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders, such as pathogens and foreign substances. This unit covers the immune system\'s structure and function, focu...

**The Immune System: Humoral and Cell-Mediated Immunity** The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful invaders, such as pathogens and foreign substances. This unit covers the immune system\'s structure and function, focusing on both humoral and cell-mediated immunity. It also explores the concepts of active and passive immunity, the innate immune system, and the anatomy of the immune system. **1. Types of Immunity** **Active Immunity:** - **Natural Active Immunity:** This occurs when a person is exposed to a pathogen, leading to the activation of the immune system and the production of memory cells. For example, recovering from chickenpox provides long-lasting immunity against the disease. - **Artificial Active Immunity:** This is acquired through vaccination, where a person is exposed to a weakened or inactivated form of a pathogen, prompting the immune system to produce a response and create memory cells without causing illness. **Passive Immunity:** - **Natural Passive Immunity:** This occurs when antibodies are transferred from one individual to another. An example is the transfer of maternal antibodies to the fetus through the placenta or to the infant through breast milk. - **Artificial Passive Immunity:** This involves the injection of antibodies or immune globulins from an immune individual into a non-immune person. For instance, administering antivenom to a person bitten by a snake provides immediate but temporary protection. **2. Innate Immune System** The innate immune system is the body\'s first line of defence against pathogens. It is non-specific, meaning it responds to pathogens in a general way and does not provide long-lasting immunity. **Anatomical Barriers:** - **Skin:** The skin acts as a physical barrier, preventing the entry of pathogens. It is composed of tightly packed cells and is covered with a layer of acidic sebum, which inhibits microbial growth. - **Mucous Membranes:** These line the respiratory, gastrointestinal, and urogenital tracts, trapping pathogens in mucus, which is then expelled from the body. - **Cilia:** Hair-like structures in the respiratory tract that sweep mucus and trapped particles out of the airways. **Inflammation:** - **Process:** Inflammation is a rapid response to injury or infection, characterised by redness, heat, swelling, and pain. It involves the recruitment of immune cells to the affected area to eliminate the pathogen and begin tissue repair. - **Key Cells:** Neutrophils and macrophages are the primary cells involved in the inflammatory response. They perform phagocytosis, engulfing and destroying pathogens. **Complement Cascade:** - **Function:** The complement system is a group of proteins that work together to destroy pathogens. When activated, these proteins enhance the ability of antibodies and phagocytic cells to clear microbes, promote inflammation, and attack the pathogen's cell membrane. **White Blood Cells (Leukocytes):** - **Neutrophils:** The most abundant white blood cells, essential for early defence against bacterial infections. - **Eosinophils:** Involved in combating parasitic infections and allergic reactions. - **Basophils and Mast Cells:** Release histamine and other mediators, playing a role in inflammation and allergic responses. - **Monocytes/Macrophages:** Phagocytic cells that engulf and digest pathogens and dead cells. Macrophages also present antigens to T cells. - **Dendritic Cells:** Act as antigen-presenting cells (APCs), capturing antigens and presenting them to T cells to initiate the adaptive immune response. **3. Anatomy of the Immune System** The immune system is supported by specialised tissues and organs that facilitate the production, maturation, and activation of immune cells. **Lymphatic System:** - **Function:** The lymphatic system transports lymph, a fluid containing immune cells, throughout the body. It plays a key role in filtering out pathogens and facilitating immune responses. **Central Lymphoid Tissue:** - **Bone Marrow:** The primary site of haematopoiesis, where all blood cells, including red blood cells, white blood cells, and platelets, are produced. It is also where B cells mature before they enter the bloodstream. - **Thymus:** Located in the chest, the thymus is where T cells mature. Immature T cells (thymocytes) migrate from the bone marrow to the thymus, where they undergo selection processes to ensure they can recognise self-MHC molecules and respond to antigens. **Peripheral Lymphoid Tissue:** - **Lymph Nodes:** Small, bean-shaped structures that filter lymph and trap pathogens. They serve as a meeting ground for immune cells and antigens, facilitating antigen presentation and the activation of lymphocytes. - **Spleen:** The spleen contains red pulp, which filters the blood and removes old or damaged red blood cells, and white pulp, which contains immune cells that respond to blood-borne pathogens. - **Mucosa-Associated Lymphoid Tissue (MALT):** Found in the mucous membranes of the gastrointestinal tract, respiratory tract, and urogenital tract. MALT includes lymphoid structures such as the tonsils, adenoids, and Peyer's patches, which help protect mucosal surfaces from pathogens. - **Gut-Associated Lymphoid Tissue (GALT):** A component of MALT, GALT is found in the gastrointestinal tract and includes the tonsils, adenoids, Peyer's patches in the small intestine, and lymphoid tissue in the oesophagus. GALT plays a critical role in defending against ingested pathogens. **4. General Concepts in Immunology** **Memory:** - **Function:** The adaptive immune system has the ability to remember previous encounters with pathogens. This immunological memory allows for a faster and more effective response upon subsequent exposures to the same pathogen. **Major Histocompatibility Complex (MHC):** - **MHC Class I:** Found on all nucleated cells, MHC Class I molecules present endogenous antigens (from within the cell) to CD8+ cytotoxic T cells, which then destroy infected cells. - **MHC Class II:** Found on antigen-presenting cells (such as macrophages, dendritic cells, and B cells), MHC Class II molecules present exogenous antigens (from outside the cell) to CD4+ helper T cells, which then coordinate the immune response by activating other immune cells. **Antigen and Epitope:** - **Antigen:** A substance that is recognised by the immune system as foreign and capable of provoking an immune response. Antigens are typically proteins, polysaccharides, or lipids found on the surface of pathogens or cells. - **Epitope:** A specific part of the antigen that is recognised by an antibody or T cell receptor. A single antigen can have multiple epitopes, each capable of binding to different antibodies or receptors. **Antibody (Immunoglobulin, Ig) and Paratope:** - **Antibody:** A protein produced by B cells in response to an antigen. Antibodies have a unique structure that allows them to bind specifically to their corresponding antigen, neutralising it or marking it for destruction by other immune cells. - **Paratope:** The part of the antibody that binds to the antigen's epitope. The paratope is highly specific to its corresponding epitope, allowing for precise immune responses. **5. Humoral Immunity** **Humoral Immunity:** - **Overview:** Humoral immunity is mediated by B cells and involves the production of antibodies that circulate in the blood and lymph to target and neutralise pathogens. - **B Cell Activation:** When a B cell encounters its specific antigen, it binds to the antigen via its B cell receptor (BCR). This interaction, along with signals from helper T cells, activates the B cell to proliferate and differentiate into plasma cells and memory B cells. - **Plasma Cells:** These cells are the effector form of B cells and are responsible for producing and secreting large amounts of antibodies specific to the encountered antigen. - **Memory B Cells:** These cells persist in the body long after the initial infection has cleared. They allow for a faster and more robust antibody response if the same antigen is encountered again. **Antibody Functions:** - **Precipitation:** Antibodies bind to soluble antigens, causing them to clump together and precipitate out of solution, making it easier for phagocytes to engulf them. - **Agglutination:** Antibodies cause pathogens with antigens on their surfaces to clump together (agglutinate), which enhances phagocytosis. - **Neutralisation:** Antibodies bind to pathogens or toxins, preventing them from interacting with host cells and neutralising their harmful effects. - **Inflammation:** Antibodies can activate the complement system, leading to the release of inflammatory mediators that enhance the immune response. - **Complement Activation:** The binding of antibodies to antigens can trigger the complement cascade, resulting in the lysis of the pathogen or the enhancement of phagocytosis through opsonisation. - **Opsonisation:** Antibodies coat the surface of pathogens, marking them for phagocytosis by macrophages and neutrophils. **6. Cell-Mediated Immunity in Detail** **Cell-Mediated Immunity:** - **Overview:** Cell-mediated immunity is primarily mediated by T cells and is crucial for defending against intracellular pathogens, such as viruses and some bacteria, as well as for destroying cancer cells and managing transplant rejection. - **T Cell Activation:** T cells are activated when they recognise antigens presented by MHC molecules on the surface of antigen-presenting cells. This activation requires both the antigen-MHC complex and co-stimulatory signals. **Types of T Cells:** - **Cytotoxic T Cells (CD8+):** These cells recognise antigens presented by MHC Class I molecules on infected or abnormal cells. Once activated, cytotoxic T cells kill these cells by inducing apoptosis (programmed cell death) through the release of perforin and granzymes. - **Helper T Cells (CD4+):** These cells recognise antigens presented by MHC Class II molecules on antigen-presenting cells. Once activated, helper T cells secrete cytokines that help activate other immune cells, including B cells, cytotoxic T cells, and macrophages. - **Regulatory T Cells:** These cells play a role in maintaining immune tolerance and preventing autoimmune reactions by suppressing the activity of other immune cells. **Memory T Cells:** - **Function:** Like memory B cells, memory T cells persist after an infection has been cleared and can respond more rapidly upon re-exposure to the same antigen. **Multiple Choice Questions (MCQs)** 1. **Which of the following is a characteristic of natural active immunity?** - a\) Acquired through vaccination - b\) Involves the transfer of antibodies from mother to child - c\) Results from exposure to a pathogen and the subsequent immune response - d\) Provided by the administration of immune globulins 2. **Which cell type is primarily responsible for the production of antibodies?** - a\) T cells - b\) Neutrophils - c\) B cells - d\) Macrophages 3. **What role do MHC Class II molecules play in the immune response?** - a\) Present antigens to cytotoxic T cells - b\) Present antigens to helper T cells - c\) Activate the complement system - d\) Bind directly to pathogens 4. **Which of the following is not a function of antibodies?** - a\) Neutralisation of pathogens - b\) Activation of the complement cascade - c\) Direct killing of infected cells - d\) Opsonisation of pathogens 5. **Where do T cells mature?** - a\) Bone marrow - b\) Spleen - c\) Thymus - d\) Lymph nodes **Clinical Cases** **Case 1: Primary Immunodeficiency Disorder** **Presentation:**\ A 5-year-old child presents with recurrent respiratory infections, slow growth, and poor response to vaccinations. Blood tests reveal low levels of immunoglobulins and a reduced number of B cells. **Discussion:** - **Question:** Explain how a deficiency in B cells could lead to the symptoms observed in this child. What types of infections are they most at risk for, and how could their condition be managed? - **Answer:** B cell deficiency leads to reduced antibody production, which impairs the child's ability to fight off extracellular pathogens, particularly bacteria and viruses. The child is most at risk for recurrent bacterial infections, especially in the respiratory tract. Management may include regular immunoglobulin replacement therapy and prophylactic antibiotics. **Case 2: Organ Transplant Rejection** **Presentation:**\ A 45-year-old man who recently received a kidney transplant begins to show signs of graft rejection, including fever, pain over the graft site, and decreased urine output. A biopsy of the transplanted kidney shows infiltrating T cells. **Discussion:** - **Question:** Discuss the role of cell-mediated immunity in transplant rejection. How might this patient's immune response be modulated to prevent further rejection? - **Answer:** The patient's immune system recognises the transplanted kidney as foreign due to differences in MHC molecules, leading to an immune response primarily mediated by cytotoxic T cells. These T cells attack the graft, causing tissue damage. Immunosuppressive drugs, such as calcineurin inhibitors or corticosteroids, may be used to reduce T cell activity and prevent further rejection.