Characterization of complement-fixing activity and cross-idiotypes of rheumatoid factors in idiopathic mixed cryoglobulinemia.

The complement-fixing activity, specificity for IgG, and cross-idiotypes of rheumatoid factors (RF) isolated from the mixed cryoglobulins of nine patients with idiopathic mixed cryoglobulinemia were determined. All were IgM kappa and fixed complement. The RF complement-fixing activity did not correlate with hypocomplementemia or disease manifestations. The majority of RF belonged to the same cross-idiotype group. The RF from two of three patients with predominantly neurologic manifestations did not have any of the known cross-idiotypes and reacted better with rabbit IgG than with human IgG. The patients with neurologic manifestations also differed in that their serum complement was normal.