Immunology: Innate and Adaptive Immunity, Immunodeficiency, and Immunomodulation
Core concepts
The immune system is organized into innate (fast, non-specific: barriers, phagocytes, complement, NK cells) and adaptive (slow, specific, memory: T and B lymphocytes) arms. Candidates must understand antigen presentation via MHC class I (to CD8+ cytotoxic T cells) and MHC class II (to CD4+ helper T cells), immunoglobulin isotypes and their functions (IgM first responder, IgG opsonizer and placental transfer, IgA mucosal, IgE allergic/antiparasitic), and complement pathways (classical, lectin, alternative) with their effector consequences.
Immunodeficiency patterns
Recognize classic presentations: B-cell/antibody defects (e.g., X-linked agammaglobulinemia, common variable immunodeficiency) present with recurrent sinopulmonary infections by encapsulated bacteria; T-cell defects (e.g., DiGeorge syndrome) present with viral, fungal, and opportunistic infections; phagocyte defects (e.g., chronic granulomatous disease) with catalase-positive organisms; complement defects with encapsulated organisms and, for terminal components, recurrent Neisseria. Severe combined immunodeficiency presents early with failure to thrive and opportunistic infection.
Traps
Questions test which arm is defective based on the organism and timing. Another trap is confusing MHC I versus II restriction, or misassigning immunoglobulin function. Watch for distractors that suggest live vaccines in severe T-cell or combined immunodeficiency, which are contraindicated.
Worked rule of thumb
Ask: what organism and what age? Encapsulated bacteria in an older child with normal T cells points to antibody deficiency; opportunistic and viral infections in infancy point to T-cell or combined defects. Match the defect to the pathogen class before choosing a test.