T cells play a key role in adaptive immunity reactions, recognizing antigens using variable TCRs. Functional TCR subunit genes are formed by somatic rearrangement, and some of the resulting TCRs recognize autoantigens, the body's own molecules. The autoreactive T cells that carry such TCRs pose a threat of inducing immune reactions against their own organism. In the course of the immune system's development, some autoreactive T lymphocytes are eliminated by apoptosis, some differentiate into immunosuppressive regulatory T cells, which support immunological tolerance to autoantigens, and the rest fall into a non-functional state of anergy. Suppression of effector T cells by regulatory T cells is mediated by immunosuppressive cytokines and costimulatory molecules, depletion of stimulating IL-2, removal of autoreactive peptides together with MHC molecules, and in other ways. Impairment of self-tolerance leads to autoimmune diseases. However, the loss of immunological tolerance can be employed in tumor treatment, allowing immunotherapy and the use of the potential of autoreactive effector T cells. The fact that the efficacious immunotherapy of tumors is often accompanied by adverse autoimmune reactions currently seems to be the inevitable price paid by using this approach.