Seroconversion is the development of detectable specific antibodies to microorganisms in the blood serum as a result of infection orimmunization. Serology (the testing for antibodies) is used to determine antibody positivity. Prior to seroconversion, the blood test isseronegative for the antibody; after seroconversion, the blood test is seropositive for the antibody.
The immune system maintains an “immunological memory” against past 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 indicatesimmunosuppression 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 orimmunization.