Opponents of vaccination often claim that natural immunity (from an infection) is better than artificial immunity (from a vaccine). But opponents ignore the fact that infection always has a much greater risk of causing harm. There are two functions of the immune system: immediate response and long-term response. The immediate response kills infectious germs and promotes recovery from the infection. The long-term response maintains immunity so that the person will be protected against infection if exposed to the germs in the future. Immediate response. The parts of the immune system involved in destroying bad germs are called antibodies and lymphocytes. Antibodies are proteins made by immune cells. The antibodies attach to the surface of the germ and kill it either by damaging it directly or by allowing other white blood cells to kill the germ. Lymphocytes (white blood cells) can attack some germs directly. Usually, though, lymphocytes work indirectly by killing the cells that are infected with the germ. Unfortunately, it takes time for the body to develop an immune response. Sometimes, the infection kills the person before the immune response kicks in. Vaccines are used to stimulate both antibody and lymphocytes so that they are present in the body before exposure to an infection occurs. The vaccine fools the immune system into thinking infection has occurred. In response, the immune system starts working. There may be a difference in the amount of antibody made after infection compared with after vaccine, but the same kinds of antibody and immune cells are made. The antibodies and lymphocytes produced in both cases are aimed at the identical chemicals on the surface of the germ. Long-term response. The second function of the immune system is to establish immune memory. When we say a person is immune to a certain disease, we mean that immune memory against that particular infection has been established. Special lymphocytes, called memory cells, are stimulated by both infection and vaccination. These memory cells live (remain active) for a very long time, perhaps even for life. If a person with established immunity to a certain infection is exposed to that infection again, the active memory cells respond very quickly and signal both the cells that make antibody and the cells that attack germs to get to work. It is important to note, however, that each type of infectious germ is attacked by a separate, distinct set of antibodies and lymphocytes. Immunity to one infection does not "create" immunity to other infections. Antibodies and lymphocytes made in response to measles infection or measles vaccine react only to measles virus. Memory cells for rubella infection will not activate antibody to fight against diphtheria.

This document was published by the Canadian Paediatric society, 1997.

Posted by the Canadian Paediatric Society, July 1997.

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