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: MHC I×CD8=8MHC II× 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.
Innate/adaptive responses and immune cell functions
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CD4=
8
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.