Rheumatoid Factor (RF) is a type of protein that sometimes mistakenly attacks healthy cells and tissues in the body. RF is not always present in the blood. The presence of symptoms and elevated RF levels in the blood help diagnose rheumatoid arthritis. This condition can damage your joints, causing pain, swelling, and stiffness. It is a chronic (long-lasting) disease that has the potential to destroy joints, causing pain, swelling, and stiffness, and may also affect other organs in the body.