Failure or loss of ability by a body to recognize an antigen, particularly self antigens.
Immune system to be effect must be able to distinguish between pathogens and the body's own tissues. This not so easily accomplished "trick" is achieved via a variety of strategies, one of which involves the elimination (e.g., clonal deletion) of lymphocyotes that are able recognize self antigens. The result is what is dubbed as immune tolerance. Autoimmunity, by contrast, represents a failure of immune tolerance.
Immune tolerance also can be acquired to non-self antigens, most notably as occurs during pregnancy where the mother achieves a tolerance to antigens associated with the fetus she is carrying.
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