Exposure to bacteria, fungi, viruses, and parasites induces production of antibodies that either can be identified only during acute disease or can remain identifiable for many years. Exposure can be through immunization, from previous infection so minimal that it passed unrecognized, or from current symptomatic or prepathogenic infection. Detection and identification of specific antibodies in the blood by assays performed in the serology laboratory are preferred for obtaining diagnostic information. This is especially true when the antigen assays or culture techniques performed in the microbiology laboratory are ineffective in producing a causative agent or in clients who cannot tolerate the invasive procedure necessary to collect a specimen for culture.
Various methods for detection of antibodies are used. They include immunoprecipitation, complement fixation, neutralization assay, particle agglutination/agglutination inhibition, immunofluorescence assay, enzyme immunoassay, and radioimmunoassay. The concentrations of antibody are referred to as the titer, and their predictable patterns are useful in both diagnosing a disease and monitoring its course.