Immunoglobulin A (IgA) and IgG serum antibodies to mycobacterial antigens in Crohn's disease patients and their relatives.

Sera from patients with Crohn's disease, their relatives, their spouses, and unrelated healthy controls were assayed by enzyme-linked immunosorbent assay for immunoglobulin G (IgG) and IgA antibodies to Mycobacterium tuberculosis, M. avium, and M. gordonae. The patients had significantly higher IgA responses to mycobacterial antigens than did either their relatives or the controls. On the other hand, both the patients and their relatives had significantly higher IgG responses against these antigens than did the controls. The elevated IgA response was more pronounced against isopentanol-extracted whole bacterial cells than it was against soluble protein extracts, and it appeared to be directed against fixed surface antigens that lie under the loosely bound peptidoglycolipid or glycolipid antigens of mycobacteria.