Immunology Basics

Innate vs. Adaptive Defense

• Barriers (skin, mucus, stomach acid, lysozyme) come first. Innate responders act within hours without specificity or memory: neutrophils (most abundant, first phagocytes), macrophages (phagocytosis plus antigen presentation), NK cells, and complement (opsonization by C3b, the membrane attack complex, inflammation).
• Adaptive immunity is slow, antigen-specific, and remembers: humoral B cells become antibody-secreting plasma cells; cell-mediated T cells coordinate and kill.

MHC and T Cells

• Rule of 8: $\text{MHC I} \times \text{CD8} = 8 \qquad \text{MHC II} \times \text{CD4} = 8$ MHC I on all nucleated cells presents endogenous antigen to CD8+ cytotoxic T cells (perforin and granzyme); MHC II on APCs presents exogenous antigen to CD4+ helpers. T-cell activation needs both the TCR signal and costimulation, or anergy results. Viruses that hide by downregulating MHC I are killed by NK cells detecting missing self.

Antibodies, Memory, and Vaccines

• IgM leads the slow primary response; class-switched, affinity-matured IgG dominates the fast, high-titer secondary response driven by memory B cells — the entire logic of vaccination (active immunity). Maternal IgG or antiserum gives passive immunity: immediate but temporary. IgA guards mucosa; IgE handles allergy and parasites.

Key Takeaways

• Match antigen source to MHC class to T-cell type.
• IgM means recent exposure; IgG means memory.
• Active immunity builds memory; passive immunity borrows antibodies.
• Opsonization and NK surveillance bridge innate and adaptive arms.