Passive immunity involves the transfer of antibodies to an individual either naturally, through the placenta or mother's milk, or artificially, by the administration of an immune serum. Active immunity involves the individual's own response to a disease organism, either through natural contact with the organism or by the administration of an artificially prepared vaccine.

Immunology has long been a very active area of research. The above description is only the barest outline of the events that are known to occur in the immune response, and there is much still to be discovered. Some of the areas of research include autoimmune diseases, in which an individual produces antibodies to his or her own body tissues; hereditary and acquired immune deficiency diseases; the relationship between cancer and immunity; and the development of techniques for avoiding rejection of transplanted tissue.

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