Infectious Serology: antibodies, antigens and the window period

Infectious serology detects antibodies or antigens in a person's blood, indicating current or past infection, by measuring the immune response to viruses, bacteria, fungi and parasites.

Test types - **Antibody tests** detect specific antibodies (IgM, IgG, IgA). IgM usually indicates recent or acute infection; IgG suggests past exposure or chronic infection. - **Antigen tests** identify parts of the pathogen itself, indicating active infection.

Methods ELISA detects antibodies or antigens with high sensitivity and specificity; Western blot confirms positive screening results; rapid tests provide quick point-of-care answers; immunofluorescence assays detect antibodies using fluorescence microscopy.

Uses Diagnosis of diseases such as HIV, hepatitis, COVID-19 and syphilis; blood-donor screening; epidemiological studies of disease spread and prevalence; and evaluation of vaccine response.

Interpretation and limitations A positive result suggests exposure to the pathogen; a negative result may mean it is too early in infection for detection (the window period) or that there was no exposure. Cross-reactivity can produce false positives. Serology and PCR answer different questions — serology maps the immune response and stage of infection, while PCR detects the pathogen itself during the acute phase.