Rheumatoid factor (RF) is a protein that is produced when the immune system malfunctions. "Immunity vs. immunity." That is, a fragment of the body's defence - immunoglobulin G (IgG) is evaluated as a foreign substance and begins to be "attacked" by other immunoglobulins (rheumatoid factor). If IgG forms a complex with a cell, the entire complex is destroyed by rheumatoid factor. 

RF is formed in the joints and from them enters the bloodstream, where it begins to damage the walls of blood vessels. Consequently, patients with high levels of RF have a significant risk of extra-articular lesions. Thus, RF is a marker for autoimmune diseases such as systemic lupus erythematosus or rheumatoid arthritis.  RF appears in the blood on average 3 months after the onset of rheumatoid arthritis. The highest concentration of RF is seen in Sjögren's syndrome.