Immunology and Host Defense: Innate and Adaptive Systems
What you must know
Immunology questions test the division of labor: innate immunity (barriers, phagocytes, complement, NK cells, pattern recognition receptors) acts rapidly and non-specifically; adaptive immunity (B cells, T cells, antibodies, memory) is slower, specific, and generates memory. Know antibody classes (IgM first responder, IgG main systemic and placental in some species, IgA mucosal, IgE allergy and parasite defense), T cell subsets (CD4 helper, CD8 cytotoxic), and antigen presentation via MHC I (endogenous, CD8) and MHC II (exogenous, CD4). Understand vaccination principles (primary and booster, maternal antibody interference) and hypersensitivity types (I immediate IgE, II antibody-mediated, III immune complex, IV delayed T cell).
Traps
Questions love species differences in placental transfer - maternal antibody transfer timing differs among species (e.g., colostrum-dependent neonates). Another trap is confusing MHC I vs. II presentation pathways or assigning memory to innate immunity. Watch for questions that imply IgG is the first antibody (it is IgM) or that complement is adaptive (it is innate). Do not conflate passive immunity (colostrum, antiserum) with active immunity (vaccination, infection).
Rule of thumb
If the stem describes rapid, non-specific defense with no memory, it is innate; if it describes specificity plus memory, it is adaptive.
Worked example
A question asks which immunoglobulin is detected earliest after primary exposure. The answer is IgM, the first antibody produced; IgG follows and dominates the secondary response. The discriminator is kinetics, not concentration.