False-positive Rose-Waaler rheumatoid factor titre given by anti-ruminant IgM antibody

An antibody against ruminant (goat, sheep, bovine) IgM is described which also cross-reacts with rabbit IgM. The antibody was found in the serum of a patient with adult coeliac disease and a selective IgA deficiency. This patient gave a false high Rose-Waaler rheumatoid factor titre due to the presence of the serum antibody to rabbit IgM that was capable of agglutinating rabbit antibody-sensitised erythrocytes.