Immunology
Fundamentals
Antibodies have two Fab regions for antigen binding and an Fc region for effector function. IgM is a pentamer and fixes complement efficiently; IgG crosses the placenta; IgA is a dimer with J chain. The classical complement pathway is triggered by IgG or IgM, the alternative pathway by microbial surfaces, and the lectin pathway by mannose-binding lectin.
Hypersensitivity
Type I is IgE-mediated (anaphylaxis, atopy); type II is antibody against cell-bound antigen (autoimmune hemolysis, Goodpasture); type III is immune complex disease (SLE, post-streptococcal glomerulonephritis); type IV is T-cell mediated and delayed (contact dermatitis, TB skin test).
Serologic methods
ELISA, immunofluorescence, agglutination, nephelometry, and chemiluminescence each have distinct interference profiles. ANA patterns suggest specificity: homogeneous points to dsDNA, speckled to extractable nuclear antigens, nucleolar to scleroderma, centromere to CREST. c-ANCA (PR3) is associated with granulomatosis with polyangiitis; p-ANCA (MPO) with microscopic polyangiitis.
Traps
- Prozone (antibody excess) gives a false-negative agglutination or ELISA; repeat on a dilution.
- Postzone (antigen excess) also causes false negatives.
- IgM usually indicates recent or acute infection; IgG indicates past infection or immunity, and paired sera showing a fourfold rise are more informative than a single titer.
- Heterophile antibodies and rheumatoid factor cause false positives in sandwich immunoassays.
- A positive ANA is common in healthy people; titer and pattern matter more than the result alone.
Rule of thumb
Unexpectedly negative serology in a strongly suspected case should prompt a dilution series to rule out prozone before repeating the assay.