Practice 5 structured short- and long-answer medical questions on Weekly Year 2: Cellular Immunology Exam - May 29, 2026 (Sections B & C). Review model answe...
Answer: B lymphocytes are primarily responsible for humoral immunity. They recognize extracellular antigens via their B cell receptors (BCRs), differentiate into plasma cells, and produce antibodies. Antibodies neutralize pathogens, mark them for destruction by other immune cells, and activate the complement system. T lymphocytes are primarily responsible for cell-mediated immunity. They recognize intracellular antigens presented by MHC molecules on other cells via their T cell receptors (TCRs). Helper T cells (CD4+) assist other immune cells (like B cells and macrophages), while cytotoxic T cells (CD8+) directly kill infected or cancerous cells.
Answer: MHC molecules are cell surface proteins that present peptide fragments (antigens) to T cells. MHC class I molecules are found on most nucleated cells and present intracellular antigens (e.g., viral peptides) to CD8+ cytotoxic T lymphocytes. MHC class II molecules are found primarily on antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells, and present extracellular antigens (e.g., bacterial peptides) to CD4+ helper T lymphocytes. This presentation is crucial for initiating adaptive immune responses.
Answer: Cytokines are small, secreted proteins that act as signaling molecules between immune cells, and between immune cells and other cells. They mediate and regulate a wide range of immune responses. Key functions include: 1. **Communication/Signaling:** They allow immune cells to communicate with each other, coordinating responses. 2. **Inflammation:** Many cytokines induce or amplify inflammatory responses. 3. **Cell proliferation and differentiation:** They promote the growth and specialization of immune cells (e.g., T cell activation and B cell differentiation). 4. **Chemotaxis:** They attract immune cells to sites of infection or inflammation.
Answer: The **innate immune system** is the body's first line of defense and is non-specific. Key characteristics include: 1. **Rapid response:** It acts immediately or within hours of encountering a pathogen. 2. **No memory:** It does not develop immunological memory; each encounter is treated as a new threat. 3. **Pre-formed components:** Relies on physical barriers, chemical mediators, and specialized cells (e.g., phagocytes, NK cells) that are always present or quickly mobilized. The **adaptive immune system** is highly specific and develops immunological memory. Key characteristics include: 1. **Slow, but specific response:** It takes days to develop but targets specific antigens. 2. **Immunological memory:** It remembers past encounters, leading to a faster and stronger response upon re-exposure. 3. **Clonal expansion and differentiation:** Involves lymphocytes (B and T cells) that undergo specific activation and proliferation.
Answer: T lymphocyte activation is a multi-step process that requires both antigen-specific signaling and co-stimulatory signals to ensure a productive and safe immune response. 1. **Antigen Recognition:** This is the primary signal (Signal 1). Naive T cells encounter antigen-presenting cells (APCs) such as dendritic cells, macrophages, or B cells in secondary lymphoid organs. The APC processes the antigen and presents peptide fragments bound to MHC molecules on its surface. T helper (CD4+) cells recognize antigens presented by MHC class II molecules, while cytotoxic T (CD8+) cells recognize antigens presented by MHC class I molecules. The T cell receptor (TCR) on the T cell binds to the peptide-MHC complex. 2. **Co-stimulation (Signal 2):** For full activation, T cells require a second signal provided by co-stimulatory molecules on the APC interacting with their corresponding receptors on the T cell. The most critical pair is B7 molecules (e.g., CD80, CD86) on the APC binding to CD28 on the T cell. This co-stimulatory signal is typically absent on resting APCs but is upregulated upon detection of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs) on APCs, indicating an infection. Without co-stimulation, antigen recognition alone can lead to T cell anergy (unresponsiveness) or even induction of regulatory T cells, preventing autoimmunity. 3. **Cytokine Signaling (Signal 3):** Cytokines produced by the APC and other surrounding cells (including the activated T cell itself) provide a third signal that influences the direction of T cell differentiation and proliferation. Different cytokines promote the development of distinct T helper cell subsets (e.g., Th1, Th2, Th17, Tfh, Treg). For example, IL-12 promotes Th1 differentiation, which is crucial for cell-mediated immunity against intracellular pathogens, while IL-4 promotes Th2 differentiation, important for humoral immunity against extracellular parasites. This signal shapes the effector functions of the activated T cell population. **Significance of these steps:** * **Specificity:** Antigen recognition ensures that only T cells specific for the presented antigen are activated. * **Prevention of Autoimmunity:** Co-stimulation prevents accidental activation of T cells by self-antigens presented by MHC molecules on healthy tissues. It acts as a 'danger signal' that the pathogen is present. * **Appropriate Effector Function:** Cytokine signaling directs the T cell to differentiate into the type of effector cell best suited to combat the specific pathogen encountered, ensuring an effective and tailored immune response. For instance, a bacterial infection might require a strong Th17 response to recruit neutrophils, while a parasitic infection might require a Th2 response to activate eosinophils and mast cells.