The Antigen-Antibody Reaction
The antibody that is produced in response to a specific antigen, such as a bacterial cell or a toxin, has a shape that matches some part of that antigen, much in the same way that the shape of a key matches the shape of its lock. The antibody can bind specifically to the antigen that caused its production and thereby destroy or inactivate it. Antigen-antibody interactions are illustrated and their protective effects are described in Table 13-1.
Antigen-Antibody Interactions and Their Effects
Complement The destruction of foreign cells sometimes requires the enzymatic activity of a group of nonspecific proteins in the blood, together called complement. Complement proteins are always present in the blood, but they must be activated by antigen-antibody complexes or by foreign cell surfaces. Complement is so named because it assists with immune reactions. Some of the actions of complement are:
* It coats foreign cells to help phagocytes recognize and engulf them.
* It destroys cells by forming complexes that punch holes in plasma membranes.
* It promotes inflammation by increasing capillary permeability.
* It attracts phagocytes to an area of inflammation.
Naturally Acquired Immunity
Immunity may be acquired naturally through contact with a specific disease organism, in which case, antibodies manufactured by the infected person’s cells act against the infecting agent or its toxins. The infection that triggers
the immunity may be so mild as to cause no symptoms (subclinical). Nevertheless, it stimulates the host’s cells to produce an active immunity. Each time a person is invaded by disease organisms, his or her cells manufacture antibodies that provide immunity against the infection. Such immunity may last for years, and in some cases for life. Because the host is actively involved in the production of antibodies, this type of immunity is called active immunity.
Immunity also may be acquired naturally by the passage of antibodies from a mother to her fetus through the placenta. Because these antibodies come from an outside source, this type of immunity is called passive immunity. The antibodies obtained in this way do not last as long as actively produced antibodies, but they do help protect the infant for about 6 months, at which time the child’s own immune system begins to function. Nursing an infant can lengthen this protective period because of the presence of specific antibodies in breast milk and colostrum (the first breast secretion). These are the only known examples of naturally acquired passive immunity.
Box 13-1 provides further information about the different types of antibodies.
Artificially Acquired Immunity
A person who has not been exposed to repeated small doses of a particular organism has no antibodies against that organism and may be defenseless against infection. Therefore, medical personnel may use artificial measures to cause a person’s immune system to manufacture antibodies. The administration of virulent pathogens obviously would be dangerous. Instead, laboratory workers treat the harmful agent to reduce its virulence before it is administered. In this way, the immune system is made to produce antibodies without causing a serious illness. This protective process is known as vaccination, or immunization, and the solution used is called a vaccine. Ordinarily, the administration of a vaccine is a preventive measure designed to provide protection in anticipation of invasion by a certain disease organism. Originally, the word vaccination meant inoculation against smallpox. (The term even comes from the Latin word for cow, referring to cowpox, which is used to vaccinate against smallpox.) According to the World Health Organization, however, smallpox has now been eliminated as a result of widespread immunization programs. Mandatory vaccination has been discontinued because the chance of adverse side effects from the vaccine is thought to be greater than the probability of contracting the disease. All vaccines carry a risk of adverse side effects and may be contraindicated in some cases. People who are immunosuppressed, for example, should not be given vaccines that contain live virus. Also pregnant women should not receive live virus vaccine because the virus could cross the placenta and harm the fetus.