The Human Body


When the body is first exposed to an antigen, several days pass before the adaptive immune response becomes active. Immune activity then rises, levels off, and falls. During following exposures to the same antigen, the immune system responds much more quickly and reaches higher levels. Because the first, or primary, immune response is slow, it cannot prevent disease, although it may help in recovery. In contrast, subsequent, or secondary, immune responses usually can prevent disease because the pathogen is detected, attacked, and destroyed before symptoms appear. This complete resistance to disease is called immunity and may be achieved through either active or passive immunization.

A. Active Immunization

Active immunization occurs when a person’s own immune system is activated and generates a primary immune response. Active immunization can be triggered in two ways, either by natural immunization or by vaccination.

In natural immunization, the body contracts a disease and recovers. Because a primary immune response occurs during the illness, the immune system will mount a disease-preventing secondary response every time it is subsequently exposed to the disease. Natural immunization is developed during childhood diseases, such as chicken pox. After having had the disease once, a person is no longer susceptible to it.

Vaccination is intentional immunization against a particular disease by the use of vaccines, substances that are structurally similar to the actual disease-producing agents but that do not produce disease themselves. Most vaccines take one of two forms. The first type of vaccine, such as the vaccines for tetanus and whooping cough, contains chemically killed bacteria or other pathogenic organisms. The other type, such as the oral polio vaccine, contains weakened forms of living organisms that have been genetically selected so they do not produce disease.

B. Passive Immunization

Another way to provide immunity is by means of passive immunization. Passive immunization does not engage the person’s own immune system. Instead, the individual receives antibodies that were created in another person or animal. Such antibodies can be lifesaving when a disease progresses too rapidly for natural immunization to occur. For example, if a person who has not been immunized against tetanus bacteria is exposed to tetanus, the toxin produced by these bacteria would reach a deadly level before a primary immune response could begin. Administering antibodies against tetanus toxin quickly neutralizes the toxin and prevents death.

Passive immunization has two drawbacks: First, the person does not mount an active immune response, so the immunizing effect is temporary and the person is not immune after recovery. Second, if passive immunization is used repeatedly, it occasionally produces side effects.