Immunological memory results in a secondary immune response

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The first time a vertebrate animal is exposed to a particular antigen there is a time lag (usually several days) before the B cell–produced antibody molecules and T cells specific to that antigen slowly increase. But for years afterward—sometimes for life—the immune system “remembers” that particular antigen, allowing the body to mount a faster response the next time it encounters the antigen. How does this happen?

The answer lies in the fact that activated lymphocytes divide and differentiate to produce two types of daughter cells: effector cells and memory cells.

  1. Effector cells carry out the attack on the antigen. Effector B cells, called plasma cells, secrete antibodies. Effector T cells release cytokines and other molecules that initiate reactions that destroy nonself or altered cells. Effector cells live only a few days.

  2. Memory cells (see Figure 41.7) are long-lived cells that retain the ability to start dividing on short notice to produce more effector and more memory cells. Memory B and T cells may survive in the body for decades, rarely dividing.

Effector cells and memory cells can respond to an antigen in two different ways:

  1. When the body first encounters a particular antigen, a primary immune response is activated, in which the previously unexposed lymphocytes that recognize that antigen proliferate to produce clones of effector and memory cells.

  2. After a primary immune response to a particular antigen, subsequent encounters with the same antigen will trigger a much more rapid and powerful secondary immune response. The memory cells that bind with that antigen proliferate, launching a huge army of plasma cells and effector T cells.