(a) Naturally acquired active immunity. Active immunity may be
naturally acquired either by having a subclinical infection with a particular
microorganism or by actually having the disease. During the illness, the individual
receives an antigenic stimulus which initiates antibody production against a specific
pathogen. On a subsequent exposure to the same or antigenically related pathogen,
these antibodies will assist in the body's defenses.
(b) Artificially acquired active immunity. A major role of immunologists
interested in preventing infectious diseases has been the development of vaccines or
toxoids that are used in immunization. The immunity resulting from the injection of
these immunogens is said to be of the artificially acquired type, since this is a
man-made procedure. Killed and attenuated strains of bacteria and viruses now are
widely used forms of immunization against many diseases, including tuberculosis,
mumps, poliomyelitis, yellow fever, and measles. Toxoids, which are detoxified but still
antigenically active poisons excreted by certain bacteria, are excellent antigens.
Antibodies against toxoids are fully reactive with the native toxin and provide excellent
immunity against diseases caused by toxigenic bacteria such as tetanus and diphtheria.
(2) Passive immunity. Passive immunity is another type of acquired
immunity because antibodies are involved. It differs from active immunity by the fact
that the antibodies are produced in another individual or animal and injected into the
patient, thus providing immediate protection. Although protection is provided upon
completion of the injection, the duration of passive immunization is relatively short, a
few days to several weeks, compared to years for active immunity. This is due to the
natural degradation of injected antibody from the circulation without internal
replacement. Passive immunity also may be acquired by natural means or by artificial
means.
(a) Naturally acquired passive immunity. This type of immunity is
significant mainly in the survival of the newborn infant. The infant passively acquires
antibodies from its mother. The antibodies may pass from the immune mother to the
fetus across the placental barrier. In addition, the infant may acquire these antibodies
through breast feeding. The mother's milk is rich in antibodies for a short time. Of
course, immunity is transferred only for the diseases to which the mother is immune.
Passive immunity is especially important to the newborn; newborns are incapable of
producing antibodies on their own for a few months after birth. The antibodies received
via natural transfer from the mother are relatively short-lived with protection seldom
exceeding 6 months. Fortunately, by this time the infant's immunologic system is
functional.
(b) Artificially acquired passive immunity. Antibodies that have been
produced in another individual or animal and then administered to the patient provide
this type of immunity. This method has been used extensively in the past in the
treatment of diphtheria and tetanus through the injection of antibodies produced in
horses. Before the advent of antibiotics, passively administered antibodies were used
as the treatment for pneumococcal pneumonia. Currently, passive immunization is
mainly used for prophylaxis following exposure to such diseases as rubella and
infectious hepatitis. This is usually accomplished by injecting the patient with gamma
globulin which has been extracted from the blood of immune persons. These antibodies
provide protection for a relatively short time. Other examples of this type of
immunization are injections of hyperimmune serum and antiserum.
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