Histocompatibility antigens in psoriasis, psoriatic arthropathy, and ankylosing spondylitis.

Patients with ankylosing spondylitis, psoriatic arthritis, and psoriasis alone were typed for HLA A, B, Cw, and DR antigens, and the antigen frequencies were compared with those in a normal control population and in patients with rheumatoid arthritis. Patients with psoriasis had a significantly raised frequency of Cw6. Those with arthritis in addition to their psoriasis also had raised frequencies of B27 and DR7. Patients with ankylosing spondylitis were characterised by the expected high frequency of HLA B27. Again, those with peripheral arthritis had a higher B27 and DR7 frequency than those without. DR3 is associated with the development of erosions in psoriatic arthritis.