What are cytokines?
Cytokines are proteins which are specifically designed to correspond with all the cells responsible for governing the immune response. The cells that are involved in the body’s immune response include the immune cells, lymphoid cells and inflammatory cells. When broken down to its Greek word roots, cyto– and –kinein,cytokines mean cells that move.
How do cytokines reach their target?
Cytokines are secreted by the white cells and the other cells that I mentioned above in response to different types of stimuli. As the picture below shows, they can reach their target through one of the following mechanisms:
- Autocrine action
- Paracrine action
- Endocrine action
When the cytokines reach the target, they attach to the receptors of the target cells, which cause them to activate genes within the cells’ nucleus and therefore lead to biological effects. The most common types of cytokine signalling are autocrine and paracrine: endocrine is done relatively rarely by cytokines as it is usually done by hormones, such as the ones that I have discussed in Glands in the Brain and Pituitary=Master.
Autocrine means ‘activates itself’. In this case, the cell secretes cytokines, and these cytokines attach themselves to the receptors on the outside of the same cell.
Paracrine refers to the activation of nearby cells. In this case, a cell secretes cytokines, and the signal transfers by means of diffusion to the receptors of nearby cells, causing them to become activated. It is like travelling from one suburb to the next: it is done fairly quickly.
Endocrine refers to activation of cells a great distance within the body. If we were to compare the distance on the outside of the body, it would be the equivalent of driving on a freeway to travel to a different state or country, depending on where you live (being Aussie, it would be like me travelling from South Australia to Victoria, it’s hard to appreciate how big this country is unless you’re a local or have travelled here before!) In this particular case, the cytokines would be secreted from cells in one particular part of the body and they travel through the bloodstream (the body’s highway) until they reach their destination cells at another point in the body.
What are the types of cytokines?
There are four families of cytokines, and multiple cytokines fall under these families. The four families of cytokines are as follows:
- Interleukins (ILs)
- Interferons (IFNs)
- Tumour necrosis factors (TNFs)
- Transforming growth factors (TGFs)
There are 29 known types of interleukins, ranging from IL-1 to IL-29. There are three types of interferons, and they are known as IFN-α, IFN-β and IFN-γ (interferon alpha, beta and gamma, respectively.)There are two types of tumour necrosis factors, known as TNF-α and TNF-β (tumour necrosis factors alpha and beta). There are two types of transforming growth factors, known as TGF-α and TGF-β (transforming growth factors alpha and beta).
What are the properties of cytokines?
There are five properties that are common to all cytokines: they include pleiotropy, redundancy, synergy, antagonism and cascade induction. An explanation of these terms can be found below:
Cytokines have different biological actions depending on what their target cells are. Below is an example of the different actions of the cytokine interleukin-4 (IL-4). By proliferation, I mean that the cells increase in number. By differentiation, I mean that the cells undergo a process in which they become different from the other cells around them. For more information on B cells and mast cells, you can find them in the page Intro to the Immune System.
Usually, two or more cytokines are responsible for the same function in the human body. Having a backup is handy, because if the body is unable to produce one of them, the other one can take over. Unfortunately, the same system does not apply to hormones, as diabetes mellitus shows: there is no backup hormone to take over the missing insulin. More information about this can be found in Metabolic Disorders. An example of redundancy can be found below, where three interleukins are responsible for the same function in B cells.
You may have heard the phrase ‘the whole is greater than the sum of its’ parts.’ Well, that’s synergy in a nutshell. As the Smart Art Graphic demonstrates, when two cytokines such as interleukins 4 and 5 act on the same type of target cell, the effect is more pronounced than if the two cytokines acted alone.
Antagonism entails that one cytokine is able to either inhibit or completely offset the actions of another. As the Smart Art Graphic shows: interferon gamma is able to offset the effects of interleukin 4 on B cells.
A cascade induction is a chain reaction of events that occur within the body. As the Smart Art Graphic shows, one cytokine acting on one target cell causes the release of another cytokine and this second cytokine acts on another target cell. The process repeats itself in this manner.