Immunology: Antibody Structure, Testing, and Interpretation
Core concepts
Know immunoglobulin classes and functions: IgG (most abundant, crosses placenta, secondary response), IgM (primary response, complement fixation, largest), IgA (mucosal immunity), IgE (allergy, parasites), and IgD (B-cell receptor). Understand antibody structure - two heavy and two light chains, Fab (antigen binding) and Fc (effector function) regions. Know the difference between innate and adaptive immunity and the roles of T cells (helper CD4, cytotoxic CD8).
Serologic testing
Understand the principles of ELISA, agglutination, immunofluorescence, and flow cytometry. Know the difference between sensitivity (detects true positives) and specificity (detects true negatives), and how prevalence affects predictive value. Know that a positive test in a low-prevalence population has a lower positive predictive value. Understand the concepts of prozone and postzone in agglutination testing - antibody or antigen excess can cause false-negative results.
Traps
Questions love the prozone effect (antibody excess causes false negative) and confusing sensitivity with specificity. Another trap: assuming IgM crosses the placenta (it does not - IgG does).
Worked example
A patient with suspected early infection has a negative IgM but positive IgG. In a primary response, IgM appears first; a positive IgG with negative IgM may indicate prior exposure or a later stage. Interpret with timing. Rule of thumb: IgM = recent/primary; IgG = past or later.