Adaptive & Humoral Immunity – Detailed Study Notes
MHC Molecules and Antigen Presentation
- Major Histocompatibility Complex (MHC)
- \text{MHC Class I}
- Expressed on all nucleated cells of the body.
- Presents endogenous (intracellular) antigen.
- \text{MHC Class II}
- Expressed only on professional Antigen-Presenting Cells (APCs) such as dendritic cells, macrophages, and B cells.
- Presents exogenous (extracellular) antigen.
- Receptor Pairings
- \text{CD4}^+ \;\text{T cells (Helper)} bind \text{MHC II}.
- \text{CD8}^+ \;\text{T cells (Cytotoxic)} bind \text{MHC I}.
Helper (CD4⁺) T Cells
- Require an APC to display antigen on \text{MHC II} for activation.
- Primary role: coordinate immune responses ("help" instead of kill).
- After activation they proliferate (increase in number) and differentiate into:
- Additional active helper T cells.
- Memory helper T cells that persist for rapid secondary response.
Cytotoxic (CD8⁺) T Cells
- Activation requires two signals:
- Recognition of antigen on \text{MHC I} ("learning").
- Co-stimulation that fully activates killing function.
- Upon full activation they proliferate and differentiate into:
- Effector cytotoxic T cells (immediate killers).
- Memory cytotoxic T cells (long-term surveillance).
- Mechanism of Target Cell Destruction (Cell-Mediated Immunity):
- Release Perforin → perforates target-cell membrane, increasing permeability.
- Release Granzymes through perforin channels → enter cytoplasm and trigger apoptosis.
B Lymphocytes and Antibody Production
- Originate and mature in bone marrow.
- When activated, most B cells become plasma cells that synthesize and secrete antibodies.
- Lifespan of plasma cells ≈ 5 days.
- Reside mainly in lymph nodes.
- A minority differentiate into memory B cells for long-term humoral immunity.
Antibody Titer & Memory
- Antibody titer = concentration of circulating antibodies specific to an antigen.
- Used clinically to decide if boosters are required.
- Secondary exposure → rapid, stronger response due to memory cells.
Types of Immunity
- Active Immunity: produced by direct exposure to antigen.
- Natural infection (e.g., common cold).
- Vaccination.
- Passive Immunity: antibodies obtained from another source.
- Maternal IgG across placenta; IgA in breast milk.
- Antiserum for snake venom, immune globulin infusions, whole-blood transfusion.
- Temporary: lasts days to weeks (rarely months) because transferred antibodies decay.
Ig Classes & Special Functions
- IgE is produced in response to parasitic infections and allergies.
- Primary vs Secondary Response Graph
- Primary: early peak of \text{IgM} followed by \text{IgG}.
- Secondary: much higher \text{IgG} levels, faster onset; \text{IgM} remains similar.
Antibody Actions ("NAP")
- Neutralization
- Antibody masks biologically active sites of microbes/toxins.
- Agglutination
- Antibodies cross-link large cells (e.g., mismatched RBCs) causing visible clumping.
- MCQ wording: “The clumping of foreign cells that occurs when antibodies cross-link their antigens is agglutination.”
- Precipitation
- Antibodies cross-link small soluble antigens, forming insoluble complexes that are easier for phagocytes to detect.
Complement & Opsonization (Brief Review)
- Complement: plasma proteins that enhance immunity; details not tested this session.
- Opsonization: coating of pathogen with antibody/complement to enhance phagocytosis.
Effectors in the Immune System
- In neural pathways, effectors = muscles/glands.
- In immunity, principal effectors are:
- Antibodies (with help from immune cells such as B & T lymphocytes).
Cardinal Signs of Inflammation
- Redness (Rubor)
- Heat (Calor)
- Swelling (Tumor)
- Pain (Dolor)
Core Definitions
- Antigen: any foreign substance that elicits an immune response.
Exam-style One-Liners for Quick Recall
- Activation of \text{CD4}^+ T cell: APC + MHC II + antigen.
- Activation of \text{CD8}^+ T cell: Target cell + MHC I + antigen + co-stimulus.
- Cytotoxic kill mechanism: Perforin + Granzymes → Apoptosis.
- Plasma cell: activated B cell; lives 5 days; resides in lymph nodes; secretes antibodies.
- IgE ↔ parasites & allergies.
- Agglutination = antibody-mediated clumping.
- Passive immunity = borrowed antibodies, short-lived.
- Cardinal inflammation signs = RHSP (Red, Hot, Swollen, Painful).
- Antigen = foreign trigger of immunity.