Interleukins are a group of cytokines (secreted proteins and signal molecules) that were first seen to be expressed by white blood cells (leukocytes).

The function of the immune system depends in a large part on interleukins, and rare deficiencies of a number of them have been described, all featuring autoimmune diseases or immune deficiency. The majority of interleukins are synthesized by helper CD4 T lymphocytes, as well as through monocytes, macrophages, and endothelial cells. They promote the development and differentiation of T and B lymphocytes, and hematopoietic cells.

Interleukin receptors on astrocytes in the hippocampus are also known to be involved in the development of spatial memories in mice.

The name "interleukin" was chosen in 1979, to replace the various different names used by different research groups to designate interleukin 1 (lymphocyte activating factor, mitogenic protein, T-cell replacing factor III, B-cell activating factor, B-cell differentiation factor, and "Heidikine") and interleukin 2 (TSF, etc.). This decision was taken during the Second International Lymphokine Workshop in Switzerland (27-31 May 1979 in Ermatingen, 'near' Interlaken).

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