The aim of this project was to demonstrate the presence of immune complexes (IC) in the sera of patients with Crohn's disease (CD). The integrity of the complement system (C') was also investigated and comparisons were made with similar investigations on patients with ankylosing spondylitis (AS) and rheumatoid arthritis (RA). ICs were detected in the sera of 32.2% of patients with CD, 100% of sera from patients with AS and 55% of synovial fluids from patients with RA, as shown by assessment of anticomplementary activity (ACA). The method was believed to detect small ICs. Although total haemolytic complement, as assessed by CH50 assay, was normal both C3 and C4 levels were consistently elevated in the sera of patients with CD. All these factors were depressed in patients with AS and RA. Measurement of C' activity and serum factors C3 and C4 gives a static picture of the involvement of C' in CD. A more dynamic view could be obtained by the demonstration of C3 inactivation products in the sera of patients with CD. This was demonstrated in 32.9% of sera tested. This was taken as evidence of in vivo C' activation by ICs and it was suggested that elevated C3 and C4 levels were due to alterations in catabolism or synthesis. No relationship was found between the presence of ICs and raised C' levels; these factors were not found to be related to duration of disease activity, steroid therapy or disease activity. Since no differences were observed between patients in active or quiescent phases of the disease, it was concluded that the disease is characterized by a continuing immunological process. Gel filtration and immunoglobulin analysis of ACA positive sera revealed that ACA was confined to high molecular weight fractions containing IgG and IgA. It was concluded that ICs in the sera of patients with CD were composed of IgG and possibly IgA complexed with an unknown antigen.
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