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see immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
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; vaccinationvaccination,
means of producing immunity against pathogens, such as viruses and bacteria, by the introduction of live, killed, or altered antigens that stimulate the body to produce antibodies against more dangerous forms.
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a method of creating artificial immunity in man and animals. Active and passive immunization are distinguished.

Active immunization involves injecting antigens into the body. The commonest form of active immunization is vaccination—that is, the use of vaccines, preparations obtained from microorganisms (bacteria, rickettsias, and viruses) or their metabolic products (toxins) for the specific prevention of infectious diseases in man and animals. Active immunization is produced by applying a preparation (for example, a vaccine) to the skin, by injecting it subcutaneously, intracutaneously, intramuscularly, intraperitoneally, or intravenously, or by administering it orally or by inhalation. The dose of the vaccine influences the efficacy of immunization (to a certain limit, the immunizing effect increases with increasing dosage; hence, optimum doses, determined experimentally, are used). The immunization schedule, the reactivity of the organism, and the quality of the preparation are important factors.

The antigen is usually injected several times to create immunity. A second immunization (reimmunization) is carried out no sooner than one or two weeks after the first, so that the antibodies and antibody-forming cells will not be neutralized by excess antigen and thereby reduce the strength of the growing immunity. Immunizations repeated at intervals of several months or years are very effective.

Antigens adsorbed on aluminum hydroxide, phosphates, or alums or mixed with mineral oils are frequently used for purposes of immunization. These substances, called adjuvants, intensify the immunizing effect of the antigen, bring about its gradual absorption from the injection site, and stimulate nonspecific antibody formation. Active immunization produces prolonged immunity (for a year or more) through the formation of specific antibodies and immune cells, and through the stimulation of nonspecific immunity factors.

Passive immunization is produced by injecting serum or serum fractions from the blood of immune animals and persons subcutaneously, intramuscularly, or, in certain emergencies, intravenously. Such preparations contain preformed antibodies, which neutralize toxin, inactivate the causative agent, and prevent the agent from spreading. Passive immunization creates temporary immunity (to a month). It is used to prevent disease in cases of contact with a source of infection (measles, diphtheria, tetanus, gas gangrene, plague, anthrax, influenza [seroprophylaxis] or, if the disease has already set in, to mitigate its course [serotherapy]). Sometimes combined immunization is used: first the immune serum is injected to help the patient cope with the infection, and then the vaccine is introduced in order to create a more stable immunity.

Homologous serums (that is, serums obtained from human blood) are preferred for the seroprophylaxis of infections in man. These serums generally do not provoke allergic reactions and are effective in infections (measles, infectious hepatitis) for which appropriate serums cannot be obtained from animals. Heterologous serums, or those obtained from animals (horse serum is used most frequently), may cause sensitization and provoke allergic reactions (anaphylactic shock, serum sickness). The use of heterologous serums is being curtailed. Seroprophylaxis with protein fractions (gamma globulin, polyglobulin and so forth) of human venous, placental, and abortion blood is commonly used instead.

Immunization is widely used against animal diseases caused by pathogenic viruses (foot-and-mouth disease, cattle plague), rickettsias (hydropericarditis), and bacteria (anthrax, blackleg, brucellosis). It is carried out in a planned regime, with account taken of the local conditions and the particular characteristics of the diseases.



Rendering an organism immune to a specific communicable disease.