measles virus

If Tom has previously had the measles, there should be a significant amount of IgG antibody in his blood shortly after the most recent exposure, as a result of a secondary humoral immune response. If he has not previously had the disease and is in the early stages of a primary response, his blood might show an elevated level of IgM type antibodies. These are the first to be produced in a primary response and are ultimately replaced by IgG antibodies. IgG antibodies are able to mount a more vigorous and immediate response to subsequent exposures, preventing illness.