TYPES OF IMMUNITY
If we consider the source of immunity, that is, where it comes from, we can begin with two major cate-gories: genetic immunity and acquired immunity. Genetic immunity is conferred by our DNA, and acquired immunity is developed or acquired by natu-ral or artificial means.
Genetic immunity does not involve antibodies or the immune system; it is the result of our genetic makeup. What this means is that some pathogens cause disease in certain host species but not in others.
Dogs and cats, for example, have genetic immunity to the measles virus, which is a pathogen only for peo-ple. Mouse leukemia viruses affect only mice, not people; we have genetic immunity to them. This is not because we have antibodies against these mouse viruses, but rather that we have genes that are the codes for proteins that make it impossible for such pathogens to reproduce in our cells and tissues. Monkeys have similar protective genes and proteins for the human AIDS virus; HIV does not cause disease in these monkeys. Because this is a genetic character-istic programmed in DNA, genetic immunity always lasts a lifetime.
Acquired immunity does involve antibodies. Passive immunity means that the antibodies are from another source, whereas active immunity means that the individual produces his or her own antibodies.
One type of naturally acquired passive immunity is the placental transmission of antibodies (IgG) from maternal blood to fetal circulation. The baby will then be born temporarily immune to the diseases the mother is immune to. Such passive immunity may be prolonged by breast-feeding, because breast milk also contains maternal antibodies (IgA).
Artificially acquired passive immunity is obtained by the injection of immune globulins (gamma globu-lins or preformed antibodies) after presumed exposure to a particular pathogen. Such immune globulins are available for German measles, hepatitis A and B, tetanus and botulism (anti-toxins), and rabies. These are not vaccines; they do not stimulate immune mech-anisms, but rather provide immediate antibody pro-tection. Passive immunity is always temporary, lasting a few weeks to a few months, because antibodies from another source eventually break down.
Active immunity is the production of one’s own antibodies and may be stimulated by natural or artifi-cial means. Naturally acquired active immunity means that a person has recovered from a disease and now has antibodies and memory cells specific for that pathogen. Artificially acquired active immunity is the result of a vaccine that has stimulated production of antibodies and memory cells. No general state-ment can be made about the duration of active immu-nity. Recovering from plague, for example, confers lifelong immunity, but the plague vaccine does not. Duration of active immunity, therefore, varies with the particular disease or vaccine.
The types of immunity are summarized in Table 14–2.
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