There are several types of inflammatory arthritis; the most common include rheumatoid arthritis and psoriatic arthritis. These chronic (ongoing) diseases occur as the result of inflammation of the lining (synovium) of your joints. This inflammation is called synovitis.

Rheumatoid arthritis is a progressive disease:

• The first stage is the swelling and inflammation of the synovial lining
• The second stage is rapid cell division and growth that causes the synovium to thicken
• The third stage happens when the inflamed cells release enzymes that may damage bone and cartilage

Approximately 2.1 million people in the US have rheumatoid arthritis. Anyone can be affected by the disease, including children (juvenile rheumatoid arthritis), but 70 percent of those affected are women. Symptoms usually begin between 30 and 50 years of age.

It's not known exactly what causes rheumatoid arthritis. It may in fact be a combination of several similar diseases rather than a single disease. It is known, however, that the immune system is a key player in the disease process. If you have rheumatoid arthritis, your immune system is mistaking your healthy joint tissue for a foreign invader and attacking it.