Immunology and Serology
What to master
Separate innate from adaptive immunity, then know immunoglobulin structure and function: IgG crosses the placenta and dominates the secondary response, IgM is a pentamer marking recent exposure, IgA guards mucosa, IgE arms mast cells. Trace the classical, alternative, and lectin complement pathways and what CH50 measures. Know MHC class I (all nucleated cells, CD8) and class II (antigen-presenting cells, CD4). Be able to classify hypersensitivity types I through IV with an example of each.
Transplant: HLA typing, panel-reactive antibody, and complement-dependent cytotoxicity versus flow cytometric crossmatch, with ABO compatibility as a hard requirement. Autoimmune serology: ANA titer and pattern as a screening test, with anti-dsDNA and anti-Sm specific for SLE, anti-CCP for rheumatoid arthritis, RF, ANCA patterns, anti-GBM, and anti-TPO. Immunodeficiency workup: quantitative immunoglobulins, flow cytometry for CD4, dihydrorhodamine for chronic granulomatous disease. Methods: ELISA, immunofluorescence, agglutination, nephelometry, immunofixation, immunoblot, and multiplex platforms.
Traps
Prozone (antibody excess) causes false-negative agglutination that becomes positive on dilution; the hook effect does the same in sandwich immunoassays. ANA patterns are not diagnostic alone, and recent vaccination or infection can raise IgM unexpectedly.
Worked example
An undiluted agglutination test for syphilis is nonreactive but becomes strongly reactive at 1:16. The antibody was in excess and blocked lattice formation, so the true result is positive, not negative.