Specific immunity is an immune response directed against a specific agent. Agents, such as bacteria, viruses, toxins, foreign tissue, and parasites that are recognized by the body as foreign and stimulate im-mune responses, are called antigens. Antigens may be the whole microorganism or a part of it, such as flagella, capsule, cell wall, toxins, pollen, the white of an egg, incompatible red blood cells, foreign cells, or tissue. Chemically, antigens are usually proteins, but nucleic acids, lipoproteins, glycoproteins, and large polysaccharides may all act as antigens.
Lymphocytes play a key role in the development of specific immunity. Immunity against specific threats may be either innate or acquired.
Innate immunity is genetically determined. For ex-ample, certain viruses and bacteria that affect lower animals, do not affect humans. This type of immu-nity is present even if the individual has not been pre-viously exposed to the threat. However, in diseases such as AIDS , in which all aspects of specific defense are depressed, unusual microorgan-isms may affect the individual.
Acquired immunity is not present at birth. This type of immunity is obtained later. Acquired immunity may be obtained actively or passively.
Active immunity is produced when an individualis exposed to a foreign organism. Active immunity is long-lasting and can protect the individual from the disease for a long time, even a lifetime. A person may be naturally exposed to the organism, as when a per-son has chickenpox (naturally acquired active im-munity), or he or she may be deliberately exposed toa modified or harmless organism, as in certain types of immunizations (induced active immunity). Im-munizations stimulate the individual’s immune sys-tem to develop specific defenses against harmful or-ganisms (such as polio). If the individual comes in contact with the pathogen in the future, the defense mechanism is ready.
Passive immunity is not a result of active stimu-lation of an individual’s immune system. Antibodies against specific organisms manufactured by the mother are transferred to the developing fetus in the womb. Antibodies are also secreted in breast milk and this, too, helps protect the infant. In emergency situations, as in epidemics, antibodies (produced by another person) against specific organisms may be injected into susceptible individuals to prevent them from getting infected. Many elderly people and those at higher risk are injected this way against specific organisms. For example, antibodies may be given for hepatitis B, tetanus, and anthrax. However, this type of defense is only temporary and lasts for only a short time .