Until recently the investigation of serological responses to mycobacteria in patients with
Crohn's disease has been hindered by the considerable degree of cross-reactivity between
antigens of M.
paratuberculosis, and other mycobacterial subspecies. We evaluated the serological response of
Crohn's disease patients to a recently identified species-specific 18 kDa
protease-resistant
antigen corresponding to M.
paratuberculosis bacterioferritin. The 18 kDa
antigen was purified from M.
paratuberculosis as previously described. Serum was obtained from 40 patients with
Crohn's disease, 15 with
ulcerative colitis, 25 coeliac patients, and 21 normal blood donors. Antibody levels were measured by
enzyme-linked
immunosorbent assay (ELISA), with anti-human
IgA and
IgG alkaline phosphatase conjugate. Antibody titres were expressed as the dilution giving 1/3 of the plateau binding value of a standard positive serum (MT/3). Disease activity of the
Crohn's disease cases was assessed using the Harvey-Bradshaw index. There was no statistically significant elevation of the mean
IgG or
IgA MT/3 titres of
Crohn's disease patients over controls. No patients had antibody titres greater than two standard deviations above the mean control MT/3 titres, and there was no significant correlation between
Crohn's disease activity and level of antibody titres. These findings make it unlikely that M.
paratuberculosis is of primary pathogenic importance in
Crohn's disease.