Microbiology and Immunology
Pathogens and host defense
Microbiology questions emphasize classification, virulence factors, and pattern recognition. Know major bacterial groups by Gram stain and oxygen requirement, key toxins, and classic clinical associations. Viruses are organized by genome and replication strategy; fungi by yeast versus mold forms; parasites by life cycle and vector. Immunology requires the innate and adaptive branches, complement pathways, antibody isotypes, T-cell subsets, MHC presentation, and hypersensitivity types. Vaccines test active versus passive immunity and the difference between toxoid, conjugate, and live attenuated vaccines.
High-yield traps
A common trap is confusing colonization with infection. Another is forgetting that encapsulated organisms are poorly opsonized without antibody or complement, so splenectomy and complement deficiency increase risk from encapsulated bacteria. Opsonization is a key rule: antibodies and complement tag organisms for phagocytosis. For hypersensitivity, match mechanism rather than disease name: type I is IgE and mast cells, type II is antibody against fixed antigen, type III is immune complex deposition, and type IV is T-cell mediated.
Worked example
A patient with recurrent infections by encapsulated organisms after splenectomy needs vaccination and preventive counseling. The reasoning is not that phagocytes are absent, but that splenic clearance and opsonization are impaired. The rule of thumb: encapsulated organisms require opsonization; the spleen is a critical filter.