Radioimmunoassay profile of antiglobulins in connective tissue diseases: elevated level of IgA antiglobulin in systemic sicca syndrome.

A microplate radioimmunoassay for IgG, IgA and IgM antiglobulins reactive with rabbit IgG was performed on sera from 69 patients with various connective tissue diseases. IgM antiglobulins were detected in rheumatoid arthritis, systemic sicca syndrome and some patients with systemic vasculitis. IgA antiglobulins were found in seven of 10 patients with systemic sicca syndrome and only five of 59 patients with other connective tissue diseases. There was no correlation between the levels of IgM and IgA antiglobulins in the systemic sicca syndrome or rheumatoid arthritis (r = -0.21 and 0.2 respectively). IgG antiglobulins were not detected in any serum which lacked IgM or IgA antiglobulins. IgG isolated by DEAE columns showed antiglobulin activity in six of 15 rheumatoid arthritis and two of 10 systemic sicca syndrome sera tested. Antiglobulin analysis of sera fractionated by sucrose density ultracentrifugation at neutral and acid pH enabled the size of each class of antiglobulin to be determined. In certain sera, antiglobulin activity extended into the denser region of the gradient at pH 7.2 suggesting that the antiglobulins were complexed.