Cytokines

Cytokines are tiny proteins that are secreted by both immune and non-immune cells to communicate with one another. Cytokines bind to receptors and trigger a response in the receiving cell. Oftentimes, cytokines promote activation, proliferation, and differentiation of immune cells, but they can do other things like help increase the body temperature - causing a fever.

Now, cytokines signal to other cells mainly through autocrine and paracrine signaling; but to a lesser extent, endocrine signaling can also be employed. Now, autocrine means the cell producing the cytokine is also the cell responding to the cytokine. An example is Interleukin-2, or IL-2 which is secreted by CD4+ T helper cells. IL-2 promotes the proliferation of T lymphocytes - including the CD4+ T helper cell that produced it. Paracrine means that the cytokine is produced by one cell and that it affects cells in the near vicinity. Once again, an example is IL-2 because it helps nearby CD8+ cytotoxic T cells proliferate. That’s important because the CD8+ cytotoxic T cells aren’t good at making their own IL-2. Finally, there’s endocrine, which is when the cytokine affects a cell that’s far away, perhaps in a different organ. An example would be the inflammatory triad of Interleukin 1-beta or IL-1beta, Interleukin-6 or IL-6, and Tumor Necrosis Factor-alpha, or TNF-alpha. These cytokines are produced by macrophages and dendritic cells. During acute inflammation, these cytokines travel to the liver and the brain. In response, the liver produces acute phase reactants like C-reactive protein and Mannose Binding Lectin, and the brain increases the body’s temperature, triggering a fever. At the same time, IL-1beta and TNF-alpha also help to recruit other immune cells to the site of injury, enhancing the inflammatory response.

Overall, there are 5 main classes of cytokines. The first and most varied group are the Interleukins, which are numbered in the order they were identified, so the numbers don’t tell us anything about what they do! It used to be thought that interleukins were only sent between leukocytes or white blood cells, but it turns out that they’re released and act on both leukocytes as well as non-leukocytes.