Serum antiimmunoglobulins reactive with human and rabbit IgG in rheumatoid arthritis and other conditions.

IgG, IgA, and IgM antiimmunoglobulins reactive with human and rabbit IgG were measured in patients with rheumatoid arthritis (RA), patients with rheumatic fever or osteoarthritis, and normal individuals. Values of all antiimmunoglobulins were significantly evaluated in RA patients as compared with other groups and depended upon activity and stage of the disease. IgM antibodies with specificity for human IgG predominated quantitatively over others in sera of RA patients with high titers of RF, whereas most of those reactive with rabbit IgG in latex negative or positive RA patients belonged to IgG class. The reaction with human IgG included thermostable and thermolabile IgM antiimmunoglobulins but in that with rabbit IgG only thermostable antibodies were active.