Over the past few decades, a solid body of evidence has been built up linking certain autoimmune diseases to the presence of specific major histocompatibility complex haplotypes. Major histocompatibility complex products are used by the immune system to distinguish non-self from self and are important in the recognition of foreign antigen by T cells. T cells play a number of roles in the initiation and control of the immune response, leading to the suggestion that T cells may be important in the genesis of these diseases. T cell recognition is mediated by a heterodimeric cell surface receptor, the T cell antigen receptor. A better understanding of the molecular biology of this receptor may shed some light on the cellular and molecular processes involved in the initiation and progression of these diseases.