The production of specific ANTIBODIES to antigens (see ANTIGEN) present in the body. This may happen as a result of infection by a virus, or IMMUNISATION with a VACCINE. Thus, if blood has been tested before the event there may be little or no evidence of antibody to the condition in question; but when a sample is taken 1–2 weeks later there may be a high level of antibody confirming that recent infection has taken place. Sometimes this is the only way to prove that a particular infection has occurred or that a vaccination has ‘taken’. One example of its value would be if a woman in early pregnancy thought she might have encountered RUBELLA (German measles) which can damage the fetus. If she fails to seroconvert then she has not been infected by the virus so no action is necessary or advisable