Histopathology of the synovial membrane of peripheral joints in ankylosing spondylitis

The histological features of the synovial membrane of peripheral joints in ankylosing spondylitis are similar to those seen in rheumatoid arthritis. There is intimal cell hyperplasia, a diffuse lymphocyte and plasma cell infiltrate, and formation of lymphoid follicles. Peroxidase-antiperoxidase staining shows the presence of IgG-, IgA-, and IgM-containing plasma cells in ankylosing spondylitis. The percentage of IgM-containing cells is significantly lower in ankylosing spondylitis than in rheumatoid arthritis.