An interleukin is any of a group of about 18 cytokines (glycoproteins) that act in various ways to stimulate the immune system. Its name literally means "between leukocytes". They are produced by T lymphocytes and macrophages (a type of white blood cell) in the presence of antigens or mitogens. They cause the T lymphocytes to activate and proliferate. More interestingly, researchers have discovered that these cytokines are an important component of an immune system's "memory" for invading particles.

interleukin 1 (IL1) is produced by macrophages and plays an important role in activating T lymphocytes and B lymphocytes. This and interleukin 2 are both known to be involved in achieving T-cell-mediated immunity. It is also strongly involved in inflammation and edema responses. It is responsible for causing the cartilage damage found in people with rheumatoid arthritis.

interleukin 2 (IL2) is also known as "T Cell Growth Factor". It was the first interleukin to be discovered and continues to be the most medically useful chemical in the group. It is produced by T lymphocytes that have been activated by an antigen. IL2 stimulates other T lymphocytes to activate and differentiate regardless of what specific antigen is involved.

Because it stimulates T-cell-mediated immunity, IL2 is useful for treating certain diseases. It has been approved by the FDA for the treatment of some types of cancer, mainly kidney cancer, melanomas, and lymphomas. It is awaiting approval for the treatment of HIV infections. It is given intravenously. On the downside, IL2 can worsen some autoimmune disorders as well as arthritis, psoriasis, and diabetes. Its side effects include nausea, insomnia, nerve damage, and internal organ damage.

interleukin 3 (IL3) is produced by T lymphocytes that have been activated by a mitogen. It stimulates bone marrow stem cells and mast cells and is being tested as a medication to improve bone marrow cell production in people undergoing chemotherapy.

interleukin 4 (IL4) was discovered in 1982. It is involved in the proliferation of B lymphocytes and plays an important role in the development of T lymphocytes and mast cells. It is also strongly involved in allergic responses.

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