The first time the body is exposed to a particular antigen, the resulting immune response is mild and brief. The second time that same antigen is encountered, however, the response is strong and long-lasting. This is an important aspect of the specific immune response. How strong and long-lasting the secondary re-sponse is can vary from antigen to antigen. Immunization against specific pathogens is aimed at establishing immunologic memory through repeated, controlled exposures to their antigens (with little or no chance of being in-fected).
Some of the activated B cells that are formed as part of the primary immune response become short-lived plasma cells that produce antibodies (IgM and IgG). Others persist as memory B lymphocytes. These cells are able to multiply and transform rapidly into plasma cells during a secondary response, when they produce some IgM and lots of IgG. In the activation of T lymphocytes (both helper and cytotoxic T cells), memory T cells are also formed.