Following an initial antigen challenge there is a lag phase where no antibody is detectable. Then there is a logarithmic increase in antibody levels to a plateau and there follows a decline to low or undetectable levels. Such a primary response consists predominantly of
Figure HA.26 Antibody structure
IgM with IgG present but appearing slightly later. If the individual is challenged again, then there is a shorter lag phase before antibody is detected, the rise in antibody titre is up to 10 times greater than the primary response, the plateau is more prolonged and decline slow. The secondary response consists almost entirely of IgG and the affinity of the antibodies is greater, a process called antibody maturation.
A typical example of primary and secondary antibody responses occur with a delayed haemolytic transfusion reaction. Initial exposure to the foreign red cell antigen results in no clinical effects, but low level antibody is produced and subsequently declines. Antibody screening after the decline may miss a low level antibody, but should the individual be exposed to the antigen again, a brisk secondary response occurs which results in the destruction of the antibody coated transfused red cells by the mononuclear phagocyte system. Clinically this is apparent after 4-14 days when the patient presents with anaemia, fever and jaundice.
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