IMMUNE MEMORY AND VACCINATION
Individuals who survive an infection normally become immune to that particular disease, although not to other diseases. This is because the immune system “remembers” foreign antigens, a process called immune memory. Next time the same antigen appears, it triggers a far swifter and more aggressive response than before. Consequently, the invading microorganisms will usually be overwhelmed before they cause noticeable illness.
Immune memory is due to specialized B cells called memory cells. As discussed earlier, virgin B cells are triggered to divide if they encounter an antigen that matches their own individual antibody. Most of the new B cells are specialized for antibody synthesis, and they live only a few days. However, a few active B cells become memory cells, and instead of making antibodies, they simply wait. If one day the antigen they recognize appears again, most of the memory cells switch over very rapidly to antibody production.
Vaccination takes advantage of immune memory. Vaccines consist of various derivatives of infectious agents that no longer cause disease but are still antigenic, that is, they induce an immune response. For example, bacteria killed by heating are sometimes used. The antigens on the dead bacteria stimulate B-cell division. Some of the B cells form memory cells so, later, when living germs corresponding to the vaccine attack the vaccinated person, the immune system is prepared. The makers of vaccines are constantly trying to find different ways to stimulate the immune system, without causing disease.