A successful primary immune response results in a proliferation of B-cells that have high affinities for the pathogens that caused the response. Typically these B-cells are so short-lived, that once the pathogen is eliminated, we would expect these B-cells to die out, which means that if the pathogen were encountered again, the immune system would have to go through the whole affinity maturation process again. The immune system avoids this by somehow retaining a memory of the information encoded in these adapted B-cells.
How does immune memory work?
This is currently the subject of much debate. Several theories have been proposed, including: adapted B-cells are "memory cells" that are very long-lived; residual traces of foreign proteins are retained that constantly restimulate the immune response; and the idiotypic network hypothesis..
How does immune memory help in immune response?Figure 6. Primary responses to new pathogens are slow; memory of previously seen pathogens allows the immune system to mount much faster secondary responses .
Immune memory greatly speeds up the response to pathogens that have been previously encountered. When new pathogens are encountered, the immune system mounts a primary response during which affinity maturation is used to learn the structure of the pathogens; the primary response can take some time to clear the infection (see figure 6). If the body is reinfected with a previously encountered pathogen, it will have an adapted subpopulation of B-cells to provide a very specific and rapid secondary response. This secondary response is usually so fast and efficient, that we are not aware we have been reinfected.
An important characteristic of immune memory is that it is associative: B-cells adapted to some type of pathogen A can provide a rapid secondary response not only to A, but also to some structurally related pathogen B. This associative memory underlies the concept of inoculation (see figure 7): introduction of a benign pathogen such as cowpox induces a primary response that results in memory of the cowpox. Smallpox is similar enough to cowpox to induce a secondary response, resulting in very rapid elimination of the smallpox.