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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:
    1. Recognition of antigen on \text{MHC I} ("learning").
    2. 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

  1. Redness (Rubor)
  2. Heat (Calor)
  3. Swelling (Tumor)
  4. 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.