Studies on the major histocompatibility system in patients with ankylosing spondylitis.

One hundred % of 66 patients with pelvospondylitis were found to have HL-A27, whereas the frequency of the antigen in 60 patients with sacroiliitis was 70 %. The difference in frequency of HL-A27 between the two patient groups is statistically highly significant. This implies that tissue typing could be of clinical importance for a prognostic evaluation of patients with sacroiliitis. Studies in families with several HL-A/MLC identical sibs indicated a relatively low penetrance of disease in those carrying the gene for increased susceptibility to development of AS. No association of AS to particular MLC alleles was found either in families or in several SD-2 identical AS patients.