Seroconversion is the development of detectable specific antibodies to microorganisms in the blood serum as a result of infection or immunization. Before the seroconversion point, the antigen is prevalent and the corresponding antibody is not detectable in sera. After seroconversion, the antibody becomes prevalent and the corresponding antigen is no longer detectable in sera. Antigen-antibody complexes (immune complexes) formed in an organism are quickly removed by several immune mechanisms, so only the dominant party of an immune complex is detectable in blood sera.
Window period - At the exact seroconversion time point, both free components of immunocomplex, the antigen and the antibody, are present at relatively low levels in sera. Therefore they may be undetected by corresponding diagnostic tests and gave a false-negative result for the infection.
Serology (the testing for antibodies) is used to determine antibody positivity. Serostatus is a term denoting the presence or absence of particular antibodies in an individual's blood. Prior to seroconversion, the blood test is seronegative for the antibody; after seroconversion, the blood test is seropositive for the antibody.
Seroreversion is the opposite of seroconversion. This is when the tests can no longer detect antibodies in a patient’s serum.
The immune system maintains an "immunological memory" of infectious pathogens to facilitate early detection and to confer protective immunity against a rechallenge. This explains why many childhood diseases never recur in adulthood (and when they do, it generally indicates immunosuppression or failure of a vaccine).
In the initial (primary infection) phase of the infection, immunoglobulin M (IgM) antibodies are produced and as these levels drop (and become undetectable) immunoglobulin G (IgG) levels rise and remain detectable. Upon reinfection, IgM antibodies usually do not rise again but IgG levels will increase. Thus an elevated IgM titre indicates recent primary infection, while the presence of IgG suggests past infection or immunization.
- Tantalo et al., JID 2005:191; "Treponema pallidum strain-specific differences in neuroinvasion and clinical phenotype in a rabbit model"