Each T cell recognizes a specific antigen when it is presented in complex with a major histocompatibility complex (MHC) molecule by an antigen presenting cell. This recognition is accomplished by the T cell receptors expressed on the cell surface. T cells receptors are generated by randomly shuffled gene segments which results in a highly diverse population of T cells - each with a unique antigen specificity. Subsequently, T cells with receptors that recognize the body's own proteins need to be eliminated while still in the thymus. Through the action of AIRE, medullary thymic epithelial cells (mTEC) express major proteins from elsewhere in the body (so called "tissue-restricted antigens" - TRA) and T cells that respond to those proteins are eliminated through cell death (apoptosis). Thus "AIRE" drives negative selection of self-recognizing T cells. When AIRE is defective, T cells that recognize antigens normally produced by the body can exit the thymus and enter circulation. This can result in a variety of autoimmune diseases.