Immunology / Serology
Core concepts
Serology questions test method principles and the antibody response. IgM appears first, is pentameric, and is efficient at agglutination and complement fixation; IgG is monomeric, crosses the placenta, and dominates the secondary response. Know ELISA formats (direct, indirect, sandwich, competitive), immunofluorescence patterns (ANA homogeneous versus speckled versus nucleolar), agglutination, nephelometry and turbidimetry for serum proteins, immunofixation for monoclonal gammopathy, and flow cytometry for CD markers. Screening algorithms matter: HIV antigen/antibody combination assay with differentiation, and the syphilis reverse algorithm starting with a treponemal EIA followed by nontreponemal RPR or VDRL for disease activity.
Traps
- Prozone (antibody excess) and hook effect (antigen excess) cause false-negative or falsely low results in agglutination and sandwich assays; dilution resolves both.
- Heterophile antibodies and rheumatoid factor cause false positives; blocking reagents and alternate methods help resolve them.
- Window periods matter: early HIV infection may be antibody-negative but antigen-positive.
- IgM positivity may reflect recent infection or nonspecific cross-reactivity; a fourfold rise in IgG between paired sera is more convincing.
Worked example
An RPR is reactive at 1:8 and the treponemal EIA is positive. This supports a diagnosis of syphilis. The RPR titer is then used to monitor treatment response, since treponemal tests typically remain positive for life.