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Understanding immunodiagnostics
Audience: patients-public, students-learners ยท Review state: published
Diagnosis by antibody
Immunodiagnostics uses the immune system's own specificity to detect disease: antibodies that bind one target and no other. Tests either hunt for a marker of disease using laboratory-made antibodies, or measure the patient's own antibody response to an infection.
The family includes pregnancy tests, rapid antigen tests for infections, and a wide range of blood tests for hormones, tumour markers and autoimmune disease.
The workhorse methods
ELISA, the enzyme-linked immunosorbent assay, coats a surface with a capture antibody, adds the sample, and uses an enzyme-linked detection antibody to produce a colour change proportional to the amount of target. Reference texts describe it as a highly sensitive method for quantifying proteins and peptides in biological fluids.
Lateral flow tests put the same principle on a paper strip for use outside laboratories, trading some sensitivity for speed and simplicity.
What the results mean
Detecting an antigen or marker means the target is present now. Detecting antibodies means the immune system has met the target at some point: early antibody classes suggest recent exposure, while later classes suggest past exposure or vaccination.
Timing matters enormously. Tested too early, before antibodies form, a true infection reads negative; this window period is why negative results sometimes need repeating.
Quality and limits
Every immunoassay balances sensitivity against specificity. Cross-reactivity, where an antibody binds a similar but wrong target, creates false positives; interfering substances in blood can create false negatives. Laboratories control for these with standards, replicates and confirmatory testing.
A single immunodiagnostic result is one piece of evidence. Persistent symptoms, exposure history and clinical examination complete the picture.
Limitations
General educational overview, not an interpretation of any person's test results. Timing, cut-offs and confirmation pathways differ by test and guideline; the treating clinician and current local guidance are the authority.