Antibodies are protein molecules that attach themselves to invading microorganisms and mark them for destruction or prevent them from infecting cells. Antibodies are antigen specific. That is antibodies produced in response to antigen exposure are specific to that antigen. It is important to distinguish between primary and secondary response to an antigen challenge. Primary response refers to the first time a persons is exposed to the antigen.
Secondary response refers to subsequent exposure.
Following a primary antigenic challenge there is an initial lag phase when no antibody can be detected. This is followed by phases in which the antibody titre rises logarithmically to a plateau and finally declines again as the antibodies are naturally catabolized or bind to the antigen and are cleared from circulation. The secondary response has a shorter lag phase and an extended plateau and decline. The plateau in secondary response is typically 10X greater than in primary response. Measurement Several antigenic challenges have been used in studies of humans.