Abstract
HLA B27 is associated with a number of forms of arthritis, including ankylosing spondylitis. It has been suggested that the disease is caused by Klebsiella pneumoniae and that the bacterium evokes an odd immune response because it cross-reacts with HLA B27. This proposed cross-reactivity was investigated in a number of ways. The results consistently failed to confirm evidence from cross reaction, even in antisera with activity against both HLA B27-positive lymphocytes and klebsiella. It is suggested that the anomalous anti-klebsiella response of rabbits immunised with pools of HLA B27-positive leucocytes may be caused by antigenic bacterial fragments on the cells of infected individuals.