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The association between disease and a genetic marker may be expressed either in terms of the antigen frequency in the patient, as compared to that in control groups, or as the relative risk. Svejgaard et al have defined the latter form of expression as an indication of "how many times more frequent the disease is in persons carrying the antigen as compared to individuals lacking the antigen" (Tissue Antigens 4:95, 1974). According to Svejgaard et al, relative risk calculations from previously published data show, for example, that psoriasis is approximately five times more frequent in persons possessing the HLA-B17 antigen than in persons lacking that antigen. Despite the increased relative risk, it is also clear that there is not a one-to-one association and that most people, either with or without HLA-B17 antigen, do not have psoriasis.
We therefore believe that, although it is valid to state that a B17- (or
Grumet FC, Krulig L, Farber EM, Payne RO. Histocompatibility (HL-A) Antigens and Psoriasis-Reply. Arch Dermatol. 1976;112(7):1030. doi:10.1001/archderm.1976.01630310076021