context: Juvenile rheumatoid arthritis ( JRA), often referred to by doctors today as juvenile idiopathic arthritis ( JIA), is a type of arthritis that causes joint inflammation and stiffness for more than six weeks in a child aged 16 or younger. It affects approximately 50,000 children in the United States. Inflammation causes redness, swelling, warmth, and soreness in the joints, although many children with JRA do not complain of joint pain. Any joint can be affected, and inflammation may limit the mobility of affected joints. JRA is an autoimmune disorder, which means that the body mistakenly identifies some of its own cells and tissues as foreign. The immune system, which normally helps to fight off harmful, foreign substances such as bacteria or viruses, begins to attack healthy cells and tissues. The result is inflammation -- marked by redness, heat, pain, and swelling. Researchers still don't know exactly why the immune system goes awry in children who develop JRA, although they suspect that it's a two-step process. First, something in a child's genetic makeup gives them a tendency to develop JRA. Then an environmental factor, such as a virus, triggers the development of JRA. JRA may cause fever and anemia, and can also affect the heart, lungs, eyes, and nervous system. Arthritic episodes can last for several weeks and may recur, although the symptoms tend to be less severe during later recurrent attacks. Treatment is similar to that for adults, with an additional heavy emphasis on physical therapy and exercise to keep growing bodies active. Many of the strong medicines used for adults, though, aren't usually needed for JRA. Permanent damage from juvenile rheumatoid arthritis is now rare, and most affected children recover from the disease fully without experiencing any lasting disabilities. Doctors classify three kinds of JRA, based on the number of joints involved, the symptoms, and the presence of certain antibodies (special proteins made by the immune system) in the blood. These classifications help describe how the disease will progress. Pauciarticular Pauciarticular (paw-see-are-tick-you-lar) means that four or fewer joints are involved. This is the most common form of JRA; about half of all children with JRA have this type. It typically affects large joints, such as the knees. Girls under age 8 are most likely to develop this type of JRA. Some children with pauciarticular JRA have abnormal proteins in the blood called antinuclear antibodies ( ANAs). Eye disease affects from 20% to 30% of children with pauciarticular JRA and is more common in children with abnormal ANAs. Regular preventive exams by an ophthalmologist (a doctor specializing in eye diseases) are necessary to treat serious eye problems such as iritis (inflammation of the iris or colored part of the eye) or uveitis (inflammation of the inner eye, or uvea). Many children with pauciarticular disease outgrow arthritis by adulthood, although eye problems can continue and joint symptoms may recur in some people. Polyarticular About 30% of all children with JRA have polyarticular disease, in which five or more joints are affected. The small joints, such as those in the hands and feet, are most commonly involved, but the disease may also affect large joints. Polyarticular JRA often is symmetrical - it affects the same joints on both sides of the body. Some children with polyarticular disease have a special kind of antibody in their blood called rheumatoid factor. These children often have a more severe form of the disease, which doctors consider to be similar to adult rheumatoid arthritis. Systemic Along with joint swelling, the systemic form of JRA is characterized by fever and a light pink rash, and may also affect internal organs such as the heart, liver, spleen, and lymph nodes. The systemic form, sometimes called Still's disease, affects 20% of children with JRA. Almost all children with this type of JRA test negative for both rheumatoid factor and ANA. A small percentage of these children develop arthritis in many joints and can have severe arthritis that continues into adulthood. The main difference between juvenile and adult arthritis is that some children with JRA outgrow the illness, while adults usually have lifelong symptoms. Studies estimate that by adulthood, JRA symptoms disappear in more than half of all affected children. Additionally, unlike adult rheumatoid arthritis, JRA may affect bone development as well as the child's growth. Another difference between JRA and adult rheumatoid arthritis is the percentage of people who are positive for rheumatoid factor in their blood. About 70% to 80% of all adults with rheumatoid arthritis have rheumatoid factor, but fewer than half of all children with rheumatoid arthritis are rheumatoid factor positive. The presence of rheumatoid factor indicates an increased chance that JRA will continue into adulthood.