Reactivity of IgM-rheumatoid factors of human, feline and bovine sera

The reactions of IgM-rheumatoid factors (IgM-RFs) from human, feline and bovine sera were examined with their homologous monomeric IgGs and with IgGs of other species. Cross-reactivities with IgGs of other species were observed in each separate system. By the use of an enzyme-linked immunosorbent assay (ELISA), measurable differences in reactivities were obtained. The results suggest that IgM-RFs of human and feline origin recognize a similar determinant on the Fc region of IgG. A different moiety on the latter molecule appears to be involved in the reactivity of bovine IgM-RF.