In part, this is due to the fact that the cytokines were discovered piecemeal, bit by bit. As new ones came in, there was an understandable reluctance to forge a new category for every minutely deviant molecule.
So, with this in mind, let's try to go through the basic characteristics of cytokines. Don't worry too much if your mind isn't crystalline on this topic; it's enough for doctors to simply know roughly what they're all about.
Cytokines are proteins secreted by many cell types that modulate the function of other cell types. The mass producers in this regard are activated lymphocytes and macrophages, but a bewilderingly large number of other cells put out their share too: epithelial cells and connective tissue cells, for instance.
So far, we could almost confuse cytokines for hormones, which are also chemical messengers. But hormones tend to be what we call the substances produced by a specific organ or gland specialised for this function, rather than by otherwise-engaged cells. Yes, I know there isn't a distinct border between the two in the final analysis, but work with me, ok?
The major role of cytokines seems to be that of intercellular signalling amongst immune/inflammatory cells. This is chiefly where they are created and secreted. Yet again, this aspect of the definition is ultimately inadequate, since cytokines seem to play an integral role in haematopoesis (creation of the blood cells).
Some times cytokines are subdivided into smaller groups. Names you may catch wind of include:
- lymphokines (the subset of cytokines produced by lymphocytes)
- chemokines (the cytokines that help leukocytes to home into the target area)
- interleukins (the subgroup of cytokines produced by and acting predominantly on leukocytes; the definition is so leaky that the term is now outdated)
Famous cytokines you may come across include interleukin-1 (IL-1), IL-6 and tumour necrosis factor alpha (TNF-α).
Most cell types that produce cytokines produce numerous different types. Furthermore, most cytokines have multiple effects, including stimulating cells to produce their own batch of cytokines. In all, the signal loops between the immune cells are too complicated to be intelligible by all by the bravest of researchers.
Anyway, hope that helps!