a. Primary Immune Response. A primary antibody response occurs when an
individual first encounters a foreign antigen. After antigen exposure there is a lag of
several hours to several days before antibody is detected. This latent period depends
upon the kind and amount of antigen given, the route of administration, and other host-
dependent factors. Antibody appears between the fifth and tenth day.
(1) The latent period in antibody production is due to two factors. First,
since only few cells are producing antibody at this time, it may take several days before
antibody is measurable in the blood. Second, the first antibody molecules to appear in
the blood may find residual antigen still in circulation. When these antigen and antibody
molecules combine, they will be excreted, and the antibody will not be detected.
(2) After the latent period ends, the primary antibody response becomes
detectable. The titer of antibody gradually increases over a period of a few days to a
few weeks, plateaus, and then begins to drop. The initial antibody formed in the primary
response is IgM. During the first and second week, IgM production declines. Although
the mechanism is not well understood, evidence strongly suggests that a single
precursor B cell can give rise to a clone, which can switch from IgM to IgG production.
This phenomenon is referred to as the IgM-IgG shift. IgG production declines after a
b. Secondary Immune Response. With subsequent exposure to antigen, the
antibody response differs dramatically from the primary antibody response. This
secondary response is characterized by a sharp drop in circulating antibody because it
complexes with the newly injected antigen. Immediately thereafter, a marked increase
in antibody levels becomes evident; the antibody is primarily IgG. This increase
continues for several days, and ultimately the titer far surpasses that of the primary
response. The titer also has a longer duration when compared to the primary response.
The secondary response is often called the memory, anamnestic, or booster response