Compare and contrast natural, acquired, active and passive immunity, giving
examples of each.
Naturally acquired active immunity usually follows an episode of microorganism-induced illness. When pathogenic bacteria enter the body, they come in contact with B lymphocytes which become activated and differentiate into plasma cells. The plasma cells produce antibodies with receptor sites structured to match the antigenic determinants on the surface of the bacteria. The antibodies bind to the antigens and lead them to phagocytosis and destruction. Memory cells remain after the infectious events to resist a second encounter with the same bacterium. The immunity is natural because it develops in the natural course of events. It is active because the body actively produces antibodies in response to the bacterial attack. Normally, this type of immunity, following each exposure to a different microorganism, remains for a person’s lifetime.
Naturally acquired passive immunity is achieved during fetal development. During this time, material antibodies pass from the mother’s circulation through the placenta to the fetal circulation. The antibodies remain in the fetus for a period of three to six months after birth, protecting the child from a variety of infectious diseases. Although the child’s immune system is active and developing, it is not always adequate to meet the environmental challenges. This type of immunity is natural, developing between mother and child. It is passive because the antibodies are transferred to the child from the mother; the child did not actively produce the antibodies. Since the immunity conferred is passive, the immune system of the child does not actively form activated B lymphocytes and memory cells; the immunity is short-lived.