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The value of HL-A antigens as genetic markers and epidemiologic tools in various disease states is becoming increasingly apparent. The antigens of the HL-A system (the major histocompatibility locus in man) are found on most human cells, especially the leukocytes and cells of the epidermis and the dermis. Associations have recently been found for HL-A antigens in psoriasis. atopic dermatitis, systemic lupus ervthematosus, dermatitis herpetiformis, pemphigus vulgaris,. and malignant melanoma of the skin; in several other diseases, associations with HL-A antigens are also probably present. Two possible mechanisms of association of HL-A with various diseases are (1) a close linkage between a gene governing immunologic responsiveness to a disease and HL-A genes, or (2) antigenic (chemical) cross-reaction between HL-A antigens and the etiologic agent (e.g.. streptococci or viral capsular antigen) associated with the disease. [ABSTRACT FROM AUTHOR]

The exchange of tissues, most conveniently skin, between individuals has long afforded a means of evaluating their genetic identity or dissimilarity. The validity of the procedure depends upon the fact that in nearly all vertebrate species so far studied many different histocompatibility or H-loci, distributed on different chromosomes, determine isoantigenic specificities on the surfaces of virtually all nucleate cells of the body. The grafting of living tissue cells from a donor to a recipient of the same species results in rejection of the "homograft" unless the donor and recipient are members of the same highly inbred strain or they are identical twins.