IgM rheumatoid factor and the inhibition of covalent binding of C4b to IgG in immune complexes

Work by other investigators has shown that IgM-rheumatoid factors (IgM-RF's) can impede complement mediated inhibition of immune precipitation. We examined the binding of complement component C4b to radiolabelled IgG in model immune complexes and demonstrate that IgM-RF's are capable of reducing the covalent binding of C4b to ¹²⁵I-IgG in the complexes. Reduced binding to IgG, however, may not be accompanied by binding of C4b to IgM-RF's within the complex, as we also demonstrate that IgM-RF's are relatively poor at C4b capture compared with normal IgM.