PsA are included in the group of HLA-B27-associated joint diseases, united in the group of seronegative spondylitis (SSA). At the same time, this disease differs from ankylosing spondylitis and other spondylitis with a particularly diverse clinical picture and the existence of only the syndromes inherent in it, for example, intermittent synovitis, palindromic rheumatism or mutilating arthritis.