Immunology: Antibody Function, Complement, Hypersensitivity, and Immunoassay Pitfalls
The immunology section covers antibody structure and function, complement pathways, hypersensitivity mechanisms, and the immunoassays used in serology - with emphasis on why an assay can return the wrong answer.
Antibody and complement
An IgG monomer has two Fab arms and an Fc region; IgM is a pentamer with a J chain and is a far more efficient complement activator per molecule, while IgG dominates the secondary response and crosses the placenta. Classical complement is triggered by IgG or IgM bound to antigen (C1q), alternative by microbial surfaces, and lectin by mannose patterns; CH50 screens the classical pathway.
Hypersensitivity
Type I is IgE and mast cell mediated (anaphylaxis); type II is antibody against cell-bound antigen (autoimmune hemolysis, Goodpasture); type III is immune-complex deposition (SLE, post-streptococcal glomerulonephritis); type IV is T-cell mediated (tuberculin reaction, contact dermatitis, transplant rejection).
Assay formats
Sandwich ELISA quantifies antigen using two antibodies; competitive ELISA suits small molecules such as haptens, where signal falls as analyte rises. Flow cytometry detects surface markers on single cells, and HLA typing is performed by sequence-specific primers or NGS.
Worked example
A sandwich ELISA for a tumor marker returns an unexpectedly low result. Serial dilution produces a rising apparent concentration - the classic hook effect, in which antigen excess saturates both capture and detection antibodies. Report the diluted value and never the neat result.
Traps that cost marks
Confusing prozone (antibody excess in agglutination assays) with hook effect (antigen excess in immunoassays). Rheumatoid factor and heterophile antibodies cause false positives in sandwich formats. High-dose biotin can bias streptavidin-biotin systems in either direction.