<prompt> Rheumatoid factor isIgM Ab against Fc fragment of IgGIgG Ab against Fc fragment of IgGIgM Ab against Fc fragment of IgAIgM Ab against Fc fragment of IgM  <response>: A IgM Ab against Fc fragment of IgG Rheumatoid factor is a IgM Ab against Fc fregment of IgG. Serum IgM RF has been found in 75-80 of patients with RA; therefore, a negative result does not exclude the presence of this disease. It is also found in other connective tissue diseases, such as primary Sjogrens syndrome, systemic lupus erythematosus, and type II mixed essential cryoglobulinemia, as well as chronic infections such as subacute bacterial endocarditis and hepatitis B and C. Serum RF may also be detected in 1-5 of the healthy population.Mnemonic: Man Towards Girl: IgM Ab against Fc fragment of IgG. 