RESUMO
We describe a sensitive radioassay for detecting erythrocyte-associated immunoglobulin employing the immunoglobulin-specific reagent Staphylococcus aureus protein A. The assay was used to determine the extent to which erythrocytes from patients with different connective tissue disease bind radiolabelled protein A. Increased amounts of protein A were bound by erythrocytes from patients with systemic lupus erythematosus or with progressive systemic sclerosis when compared to a control group, whereas there was no significant binding to erythrocytes from patients with rheumatoid arthritis or with polymyositis-dermatomyositis. The assay, because of its high sensitivity and of the availability of the reagent in pure form, should be useful for the investigation of cell-bound antibodies and for the differential diagnosis of connective tissue diseases.