Rheumatoid arthritis (RA) is a chronic autoimmune disorder that primarily affects the joints but can also impact other organs and systems in the body. Unlike osteoarthritis (OA), which is primarily a degenerative joint disease due to wear and tear, RA occurs when the body's immune system mistakenly attacks the synovium - the lining of the membranes that surround the joints. This leads to inflammation in the affected joints, which can cause pain, stiffness, swelling, and eventually joint damage.