Immunology and Serology
What this domain tests
This section covers antigen-antibody reactions, innate and adaptive immunity, common serological methods and the meaning of test performance.
Core rules
- Innate immunity is immediate and non-specific; adaptive immunity is specific and has memory, mediated by B cells (antibody) and T cells (cell-mediated).
- IgM appears first in infection; IgG appears later and crosses the placenta. A fourfold rise in antibody titre between acute and convalescent samples supports recent infection.
- ELISA detects antigen or antibody with an enzyme label; agglutination and precipitation detect visible antigen-antibody lattices; immunofluorescence uses a fluorescent label.
- Sensitivity is the ability to detect true positives; specificity is the ability to detect true negatives. Predictive values depend on prevalence.
- Prozone is a false negative from antibody excess; postzone is antigen excess. Dilute and repeat.
- Window period and cross-reactivity can cause false-negative or false-positive serology.
Worked example
A slide agglutination test for a patient with strong clinical evidence is negative, but the same sample becomes positive after dilution. Reasoning: antibody excess blocked lattice formation (prozone). Dilution brings antigen and antibody into the zone of equivalence, so the result becomes valid.
Traps
- Calling a highly sensitive test highly specific.
- Reading a negative rapid test during the window period as proof of no infection.
- Ignoring prozone in a strongly positive patient.
- Using unvalidated kits outside the manufacturer's instructions.