Rheumatoid factors in leprosy and parasitic diseases.

Rheumatoid factors (RFs) occur with higher frequency and in higher titres in multibacillary forms of leprosy and several parasitic diseases than in healthy controls. The selection of controls is essential in studies of this kind. They should be individuals without signs of the disease under study living under similar socioeconomic conditions as the patients in the endemic country. In three studies where this matter was considered, RFs in lepromatous leprosy and Chagas' disease reacted more strongly with rabbit than human IgG, a feature generally considered to be quite restricted to rheumatoid arthritis. RFs interfere in various test systems, particularly in inducing false positive reactions for specific IgM antibodies in parasitic and other infectious diseases. Model experiments in rats, in vitro culture studies, and observations in humans indicate that RFs may have a protective role in trypanosome infections, malaria and schistosomiasis respectively.