Immunology and Serology
Immunology and serology tests the immune response and the assays that detect it: antibody structure, complement, ELISA, agglutination, immunofluorescence and the serology of HIV, hepatitis B and autoimmune disease.
Core rules
- IgG is the main circulating antibody and crosses the placenta; IgM is a pentamer and appears first in infection; IgA is a dimer with secretory component; IgE mediates type I hypersensitivity.
- Primary response is slow and IgM-predominant; secondary response is rapid and IgG-predominant.
- ELISA: indirect detects antibody, sandwich detects antigen. Always include positive and negative controls and a cut-off.
- Agglutination tests are quick but prone to the prozone phenomenon when antibody excess prevents lattice formation; dilute the sample and repeat.
- ANA immunofluorescence patterns include homogeneous, speckled, nucleolar and centromere, each suggesting different connective tissue disease.
- HIV: a fourth-generation combined antigen/antibody assay is the screen; reactive samples need a confirmatory test, and a negative result during the window period does not exclude infection.
Worked example
A patient with a reactive HIV screening ELISA has a negative confirmatory test. The reasoning: screening assays are designed for sensitivity, so false positives occur. Repeat the algorithm, consider the window period and send an HIV nucleic acid test if acute infection is suspected. Never give a diagnosis on a single reactive screen.
Traps
Reading a prozone reaction as negative; ignoring rheumatoid factor causing false-positive IgM assays; and interpreting a single high titre as proof of active infection without paired sera.