Cytokines are little messenger molecules that are involved in tons of different physiologic and pathologic conditions. You’ll hear about them in such excruciating detail that it can be easy to lose sight of the big picture. So here’s a short review of what cytokines are and what they do.

Most cytokines are short polypeptide molecules (a few are proteins or glycoproteins). They are secreted mainly by activated lymphocytes and macrophages (but also in lesser amounts by lots of other cells in the body). They are kind of like hormones, because they are little messengers that carry a message from one place to another. Usually the message is given to a cell that’s close by (this is called paracrine action), but sometimes the message is given to a far away cell (endocrine) or even back to the cell that made the cytokine (autocrine).

Usually, there’s some external thing – like a bug, or another cytokine – that stimulates the production and secretion of cytokines. The secretion is usually short-lived (it lasts just enough to tell the other cells what to do without nagging).

Most cytokines are pleiotropic (one cytokine affects many different cells) and redundant (there are lots of cytokines that do the same thing). They are generally grouped according to their function:

1. Cytokines involved in innate immunity and inflammation. Most of these cytokines are made by macrophages, mast cells and endothelial cells.

• TNF (tumor necrosis factor) and interleukin-1 (IL-1) (activate endothelial cells)
• Chemokines (attract different kinds of leukocytes)
• IL-12 and interferon-gamma (IFN-gamma) (involved in chronic inflammation)

2. Cytokines involved in adaptive immunity. Most of these cytokines are made by T helper cells.

• IL-2 and IL-4 (tell lymphocytes to proliferate and differentiate)
• IFN-gamma and IL-5 (activate other cells).

3. Cytokines involved in hematopoiesis. Cells that produce these cytokines include endothelial cells, macrophages, and other cells of the immune system.

• colony-stimulating factors (like G-CSF, or granulocyte-colony stimulating factor) (cause hematopoietic cells to grow).

As you’ll see (if you haven’t already), the sheer number of cytokines and their attendant functions is nauseating. You’ll have to memorize at least some of them in order to get through pathology tests and boards. But for real life, perhaps the best advice is found in the following quote from Ed Uthman:

“Since all the various cytokines have never been successfully memorized, except by a few institutionalized idiots savants, I will not go into them in detail. If anyone ever asks you about cytokines and sepsis, act irritated and mumble something about TNF-alpha, and they’ll probably leave you alone.”

Nice photo of IFN-beta from Wikimedia.