Class II major histocompatibility complex and organ specific autoimmunity in man.

T helper lymphocytes only recognize the antigen to which they are directed if it is 'presented' to them by a cell expressing the same class II major histocompatibility complex (MHC) antigen as themselves. Since the target cells of most organ specific autoimmune diseases do not normally express class II MHC proteins on their surface, any cell specific surface antigens they possess may never have been presented, and therefore immunological tolerance to them may not exist. The target cells in several organ specific autoimmune diseases have been shown to express class II MHC aberrantly and this expression may have allowed presentation of such cell specific surface antigens to potentially autoreactive T helper lymphocytes. It has therefore been proposed that aberrant expression of class II MHC antigens may be an initiating factor in certain autoimmune diseases.