user: What is (are) juvenile idiopathic arthritis ?

assistant: Juvenile idiopathic arthritis refers to a group of conditions involving joint inflammation (arthritis) that first appears before the age of 16. This condition is an autoimmune disorder, which means that the immune system malfunctions and attacks the body's organs and tissues, in this case the joints.  Researchers have described seven types of juvenile idiopathic arthritis. The types are distinguished by their signs and symptoms, the number of joints affected, the results of laboratory tests, and the family history.  Systemic juvenile idiopathic arthritis causes inflammation in one or more joints. A high daily fever that lasts at least 2 weeks either precedes or accompanies the arthritis. Individuals with systemic arthritis may also have a skin rash or enlargement of the lymph nodes (lymphadenopathy), liver (hepatomegaly), or spleen (splenomegaly).  Oligoarticular juvenile idiopathic arthritis (also known as oligoarthritis) has no features other than joint inflammation. Oligoarthritis is marked by the occurrence of arthritis in four or fewer joints in the first 6 months of the disease. It is divided into two subtypes depending on the course of disease. If the arthritis is confined to four or fewer joints after 6 months, then the condition is classified as persistent oligoarthritis. If more than four joints are affected after 6 months, this condition is classified as extended oligoarthritis.  Rheumatoid factor positive polyarticular juvenile idiopathic arthritis (also known as polyarthritis, rheumatoid factor positive) causes inflammation in five or more joints within the first 6 months of the disease. Individuals with this condition also have a positive blood test for proteins called rheumatoid factors. This type of arthritis closely resembles rheumatoid arthritis as seen in adults.  Rheumatoid factor negative polyarticular juvenile idiopathic arthritis (also known as polyarthritis, rheumatoid factor negative) is also characterized by arthritis in five or more joints within the first 6 months of the disease. Individuals with this type, however, test negative for rheumatoid factor in the blood.  Psoriatic juvenile idiopathic arthritis involves arthritis that usually occurs in combination with a skin disorder called psoriasis. Psoriasis is a condition characterized by patches of red, irritated skin that are often covered by flaky white scales. Some affected individuals develop psoriasis before arthritis while others first develop arthritis. Other features of psoriatic arthritis include abnormalities of the fingers and nails or eye problems.  Enthesitis-related juvenile idiopathic arthritis is characterized by tenderness where the bone meets a tendon, ligament or other connective tissue. This tenderness, known as enthesitis, accompanies the joint inflammation of arthritis. Enthesitis-related arthritis may also involve inflammation in parts of the body other than the joints.  The last type of juvenile idiopathic arthritis is called undifferentiated arthritis. This classification is given to affected individuals who do not fit into any of the above types or who fulfill the criteria for more than one type of juvenile idiopathic arthritis.