1. The expression of leucocyte antigens CD11/CD18 and complement receptor 1 was studied on the circulating leucocytes of 13 patients with inflammatory bowel disease and 13 age- and sex-matched healthy control subjects.

2. Monoclonal antibodies against CD11/CD18 and complement receptor 1 were added to leucocyte suspensions from patients and control subjects. Antibody binding was detected using a fluorescein-conjugated rabbit anti-mouse antibody and flow cytometry. The proportions of lymphocytes, monocytes and granulocytes expressing these molecules and the density of antigen expression, measured as mean fluorescence intensity, were determined.

3. There were no differences between patients and control subjects in the mean fluorescence intensity of antibody staining of surface molecules or in the proportion of cells expressing each molecule for any cell type. Analysis of subgroups of patients according to disease type, severity or treatment also showed no difference compared with control subjects.

4. We conclude that failure to identify a population of circulating leucocytes whose adhesion molecules or complement receptors are upregulated may arise because cells are only activated locally within the gut vasculature. Alternatively, structural changes in these molecules, rather than an increase in their number or the expression of other surface glycoproteins, may be more important in mediating adhesive interactions in inflammatory bowel disease.

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