USMLE® Step 1 style questions USMLE
Content Reviewers:Rishi Desai, MD, MPH
They’re 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.
There are three main ways by which cells use cytokines to signal to other cells - autocrine, paracrine, and endocrine.
Autocrine means the cell producing the cytokine is also the cell responding to the cytokine.
IL-2 promotes the proliferation of all 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 epithelial cells that are injured or infected, and then the cytokines travel to the liver and the brain.
Overall, there are 5 types 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.
The second group are the Tumor Necrosis Factors or TNFs.
TNFs are named that because they can bind to some cell receptors on tumor cell lines and cause those cells to die.
TNF-alpha, TNF-beta, and lymphotoxin, which is sometimes called LT-beta are also heavily involved in the inflammatory responses made to pathogens.
The TNF family members are known to have a wide variety of biological effects in the inflammatory response including activating endothelial cells to upregulate expression of adhesion molecules, increasing vascular permeability, and helping to induce fever.
The third group are the Interferons, and as their name implies - they interfere with processes like viral replication.
When surrounding cells detect these type I interferons, they start trying to protect themselves.
They start to degrade mRNA which blocks viruses from rapidly making copies of themselves, inhibit protein synthesis which blocks viral proteins from being produced, and they increase expression of MHC molecules, which makes it easier for cytotoxic CD8+ T cells and NK cells to do surveillance and kill them if they’re infected.
The fourth group includes the colony-stimulating factors.
These cytokines bind to surface receptors on hematopoietic stem cells, causing them to proliferate and differentiate.
Some examples include granulocyte-Macrophage Colony stimulating factor or GM-CSF, which is a cytokine secreted by a wide variety of immune and non-immune cells, that stimulates the development of granulocytes and macrophages.
The fifth and final group includes the Transforming growth factors.
There are more than 30 different TGFs, and the most important is TGF-beta.
TGF-beta is an inhibitory factor that controls proliferation and differentiation of various cells.
You can think of TGF-beta as helping to put on the brakes.