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Immunity, system of defense in animals protecting against foreign materials, specifically, infectious microorganisms, parasites, and their products. For many diseases, humoral immunity, exposure to the causative organism in disease itself or by vaccination, provides acquired resistance to that organism, making further infection with it unlikely or less severe. The antigen (foreign microorganism or other substance) provokes the formation of an antibody specific to that antigen. The antibody tends to neutralize viruses or to bind to antigens, encouraging destruction of bacteria by white blood cells. A number of diseases are due to the systemic effects of immune complexes (antibodies linked to antigens) that arise in the appropriate response to an infection or in serum sickness, and these especially affect the kidneys, skin, and joints. In autoimmunity antibodies are produced to antigens of the body's own tissues, for reasons that are not always clear; secondary tissue destruction may occur. The second major type of immunity, cell-mediated immunity, only occurs with certain types of infection (tuberculosis, histoplasmosis, and fungal diseases) and in certain probable autoimmune diseases. It is also important in the immunity of transplants. Lymphocytes, primed by infection or by the autoimmune or graft reaction, produce substances that affect both lymphocytes and the source of infection and result in a type of inflammation with much tissue damage. Investigation of the role of immunity and its disorders in the causation and manifestations of many diseases has led to the development of immunosuppressive drugs and other agents that are able to interfere with abnormal or destructive immune responses. Immune deficiency diseases have provided models for the separate parts of the immune system, and have led to methods of replacement of absent components of immunity. The epidemic of acquired immune deficiency syndrome (AIDS) has added some urgency to research. Passive immunity, the transfer of antibody-rich substances from an immune subject to a non-immune subject who is susceptible to disease, is important in infancy, where maternal antibodies protect the child until its own immune responses have matured. In certain diseases, such as tetanus and rabies, immune serum gives valuable immediate passive protection in non-immune subjects.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Humber, River to Indus Valley civilization