Seroconversion is the development of detectable specific antibodies to microorganisms in the serum as a result of infection or immunization. Serology (the testing for antibodies) is used to determine antibody positivity. Prior to seroconversion, the blood tests seronegative for the antibody; after seroconversion, the blood tests seropositive for the antibody.
The word is often used in reference to blood testing for anti-HIV antibodies.
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 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 but the presence of IgG does not reveal whether the infection is recent or past. i.e.- The presence of IgM is a sign of current or recent infections. Presence of IgG suggests past infection or immunization.