High doses of intravenous immunoglobulin do not affect the recognition phase of the classical complement pathway

We have recently found that intravenous immunoglobulin (IVIg) prevents deposition of C3 and C4 fragments onto antibody sensitized erythrocytes. To find out if such an effect results from the blockade of the recognition phase of the classical complement cascade, we investigated the ability of human serum containing high concentrations of IVIg to deposit the recognition subunit of the first complement component (C1q) onto targets. Normal human serum supplemented in vitro with IVIg did not demonstrate reduced C1q binding to targets as determined by radiolabeled antihuman C1q antibody uptake. Similarly, methylamine-treated normal human serum to which IVIg was added was equally effective in terms of C1q binding as the same serum without IVIg. At increasing doses of sensitizing antibody, C1q uptake decreased proportionally; however, at all antibody dilution points C1q uptake was not significantly different in the serum with IVIg in comparison with normal serum. Serum from a patient treated with IVIg did not differ in its capacity to deposit C1q from the same patient's serum before therapy. Our data suggest that IVIg does not interfere with the recognition step of classical complement pathway. This is a US government work. There are no restrictions on its use.