RF is represented by immunoglobulins, mainly IgM but also IgG and IgA, directed towards the constant (Fc) portion of another immunoglobulin (101); serum RF is positive in up to 80% of patients with RA, albeit with lower specificity (101) since non-rheumatic conditions (e.g., infective endocarditis, hepatitis B and C, primary biliary cholangitis, lymphoma) and rheumatic diseases other than RA (e.g., Sjogren’s syndrome, cryoglobulinemia) manifest different degrees of RF positivity (102). Here, CD79A is linked to rheumatoid arthritis.