Juvenile arthritis (JA) is an immune disorder in which the body’s immune system mistakenly attacks its own healthy cells and tissues, causing excessive inflammation in and around the joints.
Inflammation occurs when an injury or disease stimulates the immune system to send signaling molecules and white blood cells to promote tissue repair or fight foreign invaders. In people with JA, the inflammatory response is prolonged, particularly during movement of the joints. The underlying reason for this excessive inflammatory response is not clear. Toxins, diet, allergies or emotional factors do not appear to play a role in the development of the disease.
Based on the type of immune cells that are over-activated, JA may be classified as autoimmune JA or autoinflammatory JA.
Autoimmune JA
In this case, the immune system becomes overactive and recognizes self-molecules as foreign and thus, starts producing self-attacking antibodies, or autoantibodies. The overactive immune cells are part of the adaptive immune system — the immune system that is activated by a specific antigen, or foreign molecule. They produce antibodies that attach to the antigen so that it is recognized and destroyed. It is not known what causes the activation of immune cells in response to self-molecules.
Autoinflammatory JA
In this case, the immune system is not stimulated by self-molecules to produce autoantibodies. Instead, the immune cells become self-defensive on their own due to unknown reasons. These immune cells (such as macrophages) are part of the innate immune system — the nonspecific defense mechanisms that become active within hours of a foreign attack.
Risk Factors of Juvenile Arthritis
Research suggests that genetic makeup and environmental factors, such as infections, may influence an individual’s risk of developing JA.
Scientists speculate that the development of JA is a two-step process. First, an individual inherits genes from his or her parents that increase their risk of developing JA. Then, if the individual is exposed to other triggering factors such as a virus, their high-risk genes cause the disease to develop.
Changes in several genes have been found to be associated with the development of JA. Many of these genes belong to a family of genes that code for a group of related proteins called the human leukocyte antigen (HLA) complex. The HLA complex proteins present the foreign antigens to the immune cells, allowing the immune system to distinguish the body’s own proteins from the proteins of foreign invaders (such as bacteria or viruses). Several different variations of each HLA gene exist naturally, allowing each person’s immune system to react to a wide range of foreign proteins. Certain normal variations of several HLA genes seem to influence the risk of developing JA.
JA has also been associated with normal variations in several other genes, including HLA-A, HLA-B, HLA-DPB1, HLA-DQA1, HLA-DQB1, HLA-DRB1, IL2RA, IL6, MIF, PTPN22, SLC11A1, STAT4, TNF, TNFAIP3, TRAF1, and WISP3. Many of these genes are thought to play roles in the immune system.
Inheritance
In most patients, JA develops in a sporadic manner, meaning patients have no history of the disorder in their family. Only a small percentage of JA cases have been reported to run in families, although the inheritance pattern of the condition is not clear. An individual who has a sibling with JA has a 12 times higher risk of developing the condition.
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