Rheumatoid factor is a blood test, commonly used for the diagnosis, screening and the progress of the autoimmune diseases such as rheumatoid arthritis, Sjögren's syndrome . Rheumatoid factors are proteins produced by your immune system that can attack healthy tissue in your body. High levels of rheumatoid factor in the blood are most often associated with autoimmune diseases, such as rheumatoid arthritis and Sjogren's syndrome. But rheumatoid factor may be detected in some healthy people, and people with autoimmune diseases sometimes have normal levels of rheumatoid factor.