The body’s natural protective response to injury or infection. When the body’s immune system detects an injury or infection, it responds by increasing the flow of blood and attracting various immune cells (such as white blood cells) to the injured or infected area, where they work to eliminate the problem and repair the injured tissue. However, this increased blood flow, and the subsequent enlarging of the tissues in the affected area, also brings about the pain, swelling, redness, heat, and loss of function characteristic of inflammation. These effects are usually temporary; when the infection is controlled or the injury healed, the inflammation subsides.
Sometimes, however, inflammation develops but doesn’t subside. If this inflammation is located in the body’s joints, the resulting condition is called arthritis. The word “arthritis” literally means “joint inflammation.” There are many ways in which a joint can become inflamed. In bursitis, the bursae, the small sacs of fluid in a joint that lubricate the joint’s movement, become aggravated (often by overuse) and are subsequently inflamed. In tendinitis, the tendons, tough bands of tissue that connect muscle to bone, are aggravated or injured, bringing about inflammation. In gout, the presence of uric acid crystals in the joint cartilage leads to inflammation. In osteoarthritis (OA), the breakdown of cartilage in the joints causes the bones to rub against each other and can result in inflammation, although inflammation is often less prominent in OA than in many other types of arthritis.
Inflammation also plays a leading role in autoimmune arthritis (also known as inflammatory arthritis). In autoimmune arthritis the immune system, for reasons that are not entirely understood, mistakenly attacks the tissues of the joints. This chronic inflammation can cause significant problems—in addition to pain and loss of function, there can be deterioration of cartilage and bone in the joint. Autoimmune types of arthritis include rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), psoriatic arthritis, ankylosing spondylitis, polymyalgia rheumatica, and polymyositis.
Many arthritis drugs reduce inflammation. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (Advil, Motrin) and naproxen (Aleve) are the most common ones. Corticosteroids such as prednisone replicate the body’s adrenal hormones, which help to tamp down inflammation. Disease-modifying antirheumatic drugs (DMARDs) such as methotrexate (Rheumatrex, Trexall) work by blocking immune system activity and therefore decrease inflammation. Biologic drugs, which include etanercept (Enbrel) and infliximab (Remicade), attack specific immune system molecules for a more precise effect on inflammation. DMARDs and biologics are usually only used to treat autoimmune arthritis. Corticosteroids are used to treat autoimmune arthritis and are also injected into joints to treat local inflammation from OA.