Production of white blood cells and antibodies in reaction to an invading disease organism is called an immune response. This response is one of the body's primary and most efficient lines of defense. In most cases, once antibodies have been produced to fight a certain organism, it no longer poses a great threat to the body. That is why one attack of a disease often prevents that same disease from infecting the body again -- the first attack causes production of antibodies that protect the body against subsequent attacks. With measles, for example, antibodies are produced as a result of having the disease or of being immunized with the measles vaccine. These antibodies are able to resist a second attack of the disease.
Antibodies are not always beneficial. For example, when tissue from another body, such as a transplanted heart, is introduced, antibodies are produced to destroy the "invader." Transplants usually are made possible only by means of drugs that act against the body's natural immune response. Also, when blood is transfused from one person to another, it must be of a matching type; otherwise, the recipient's immune system will manufacture antibodies to destroy the transfused blood.
Sometimes, the immune system causes reactions that make the body unusually sensitive to foreign material. When the immune response is disruptive to the body in this way, it is called an allergic reaction. Let's look at this important mechanism, and the types of allergens, in the next section.