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HISTOCOMPATIBILITY ANTIGENS (HLA) IN DERMATOLOGY

That genetic markers can indicate a predisposition to disease was suggested in the early 1950s by the finding that patients with duodenal ulcers had an increased incidence of blood group O and patients with gastric carcinoma had an increased incidence of blood group A (Aird et al., 1953. 1954) The strength of the association was weak and a stronger predictive marker was needed. This need seemed to be filled by study of the HLA system, which had developed out of the necessity to find a way of matching donors and recipients in the transplantation of tissues. Up to March 1983,611 papers have been published on HLA and skin and related diseases, but many of these have been based on small numbers or inadequate statistics. The literature up to 1978 has been reviewed by Ozawa, Ohkido and Kimiyoshi (1981). Initially the antigens fell into four series, three of which, HLA-A, -B and -C, were serologically defined, and the fourth, HLA-D, lymphocyte defined. More recently, a series of antigens has been serologically defined on lymphocytes of B-cell origin which are closely related to the D locus, and are called DR antigens. It is likely that immune response and immune suppression genes occur in the DR region of the HLA complex. In 1979 there were seventeen A, twenty-seven B, seven C, eleven D and seven DR determinants and at that time it was calculated (Bodmer, 1980) that these could give rise to 300 million genetically different individuals and 30 million distinguishable combinations of antigens. No HLA-disease association is absolute. The association of ankylosing spondylitis with B27 is most striking, but a few patients with ankylosing spondylitis are B27 negative, and many people with B27 do not get ankylosing spondylitis. Many explanations have been proposed to account for these contradictions. It is likely that non-HLA genes are also predisposing factors and that environmental factors such as infection may be important. Moreover, diseases are not necessarily homogeneous and HLA factors may be related only to certain clinical, pathological or immunological subsets of a disease. Resistance to a disease may be just as important as susceptibility. For example, it is known that HLA is associated with prognosis in Hodgkin's disease (Falk & Osoba, 1971) and acute leukaemia (Lawler et al., I974) and, in our experience, with systemic sclerosis. Diseases seem to fall into two distinct groups in their relation to HLA types (Bodmer, 19XOJ. The first group includes disorders considered to be autoimmune (I prefer the term autoaggressive) or have immunological features and these are usually associated with DR3 or B8. DR3 is in linkage disequilibrium with B8 and it is likely that the primary association is with DR3 rather than B8. Diseases in the second group are associated with an antigen, or antigens, of the HLA-A, -B or -C series, the most definite example of which is the relationship between ankylosing spondylitis and B27. It is important to realize that some associations may be primary and others secondary. For example, psoriasis is the only disease at present known to be associated with the C series. The association of psoriasis with CW6 is probably the primary association, and this induces a number of secondary associations such as B17, B13 and B37 known to be associated with CW6 in normal subjects. It is thought that CW6 in psoriasis may