Cytokines.

The term cytokine covers a large number of smallish proteins (usually less than 20 kDa) that serve a hormone-like function in enabling cells to communicate with each other. Most people are familiar with hormones such as insulin and growth hormone, which are produced in one organ or tissue and travel through the bloodstream to other organs where they bind to receptors on the cells of that organ and stimulate a particular response. Hormones that are produced in one organ and act on a distant tissue are said to be acting in an endocrine manner. Cytokines do not usually act in an endocrine manner; rather, they act locally. They are produced by cells in a particular tissue and act on ‘cells’ in that tissue. Cytokines therefore act in a paracrine or autocrine manner. Paracrine action means that the cytokine binds to receptors on cells close to those producing the cytokine; by ‘close’ we are probably talking about a microenvironment of microns to 1 mm. Autocrine means that the cytokine actually binds to receptors on the cell that produced the cytokine. Thus the role of cytokines is to enable cells to communicate with each other in a local environment. A few cytokines can also act in an endocrine manner.
There are many cytokines and they can be divided into families. The main families of cytokines are the interleukins, colony-stimulating G-CSF, granulocyte-CSF; M-CSF, macrophage-CSF; GM-CSF, granulocyte/monocyte-CSF; MCP, macrophage chemotactic protein; TGF, transforming growth factor; IGF, insulin-like growth factor.

Action of hormones. Endocrine: the hormone is secreted at one site of the body and travels through the bloodstream. The hormone will bind to receptors on cells at a distant site (blue cells) and cause a response in those cells. Paracrine: hormones produced by cells in a tissue bind to receptors on other cells in the immediate vicinity (blue cells). Cells in other parts of the same tissue are not affected by the hormone (white cells). Autocrine: the secreted hormone binds to receptors on the cell that produced the hormone (blue cell).

The functions of cytokines will be described in detail at the appropriate times when particular mechanisms are being explained. It is important to realise that in the body, cells are never exposed to a single cytokine – they will be exposed to a number of different cytokines, probably produced by a number of different cell types. Different cytokines can either act cooperatively in promoting a response or act antagonistically in inhibiting each other’s actions. It is the combination of cytokines to which a cell is exposed that determines the behaviour of the cell.