Monoclonal rheumatoid factor–igg immune complexes. poor fixation of opsonic c4 and c3 despite efficient complement activation

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Abstract
Monoclonal IgM rheumatoid factor forms complexes with IgG in essential mixed cryoglobulinemia. We demonstrate that such complexes fix C3 and C4 poorly, although efficient fluid-phase C3 conversion can occur. Fixation of small amounts of C4 may be sufficient to generate a C3 convertase, but may prevent subsequent fixation of C3 by competing for binding sites on the complex. These complexes bind inefficiently to normal erythrocyte complement receptor type 1 (CR1) in vitro, and are undetectable on erythrocytes of patients with essential mixed cryoglobulinemia in vivo. Clearance of such phlogistic complexes from tissues by CR1-bearing cells may be inefficient.