Circulating immune complexes in Crohn's disease. Their characterization and interrelationship with components of the complement system.

Of the sera from 55 patients with Crohn's disease, 37% showed evidence of anticomplementary activity, C3 and C4 levels were significantly increased in these patients compared with control subjects, although total hemolytic complement levels were normal. C3 inactivation products were demonstrated in 32.9% of serum and plasma samples from 51 patients with CD. This was taken to be evidence of in vivo activation of C3 and the complement system. Activation of the C' system was thought to be caused by circulating immune complexes since evidence of anticomplementary activity could be correlated with incidence of C3 activation. Gel filtration studies carried out on 15 serum samples (13 patients) showing anticomplementary activity showed this activity to be confined to two fractions of molecular weight range greater than 10(6) and 2 x 10(5) - 10(6) daltons. IgG and IgA were present in both these fractions. As the molecular weights of IgG and IgA are lower than the molecular weight range of these fractions, it is possible that the immune complexes were composed of IgG and possibly IgA complexed with antigen.