Pro-inflammatory Cytokines and Chemokines
The word cytokine comes from the Greek cyto-, cell and -kinos, movement. They are messenger molecules released by many cell types in response to an immune stimulus like a pathogen or even an allergen. Together they play critical roles in mediating and regulating immunity and inflammation.
So how do cytokines help with the immune response? To set the scene, take a look at this video of a macrophage detecting bacteria.
The macrophage engulfs several bacteria but there are many more around that need neutralising. The macrophage can't do this on its own. It therefore releases cytokine signals, calling for "mobilised backup". The same would happen if it had detected another pathogen like a virus or fungi.
Some of the cytokine released by immune cells help eliminating pathogens by directly acting on infected cells. For example, during a viral infection some immune cells fired out a kind of cytokine called interferon (IFN). Interferons were first described in 1957 and were so named because they "interfered" with viral functions. The type I IFNs (IFN-α and β) instruct infected cells to die by apoptosis. During apoptosis the virus is prevented from replicating and apoptosis also halts the spread of the infection. Consider what would happen if no interferon was produced and the infected cell instead died by cell lysis!
Other cytokines indirectly help eliminating pathogens by attracting and activating other immune cells. When lipid mediators cause blood vessels to dilate and histamine makes them leakier, phagocytes and lymphocytes can more easily migrate into the infected tissue. These cells are drawn closer to the infected site by the chemoattractant cytokines called chemokines release by cells like mast cells and macrophages at the infected site. Watch how respondent these neutrophils are to the chemokines in this video.
Cells at the infected site also release pro-inflammatory cytokines like IL-1, Tumour Necrosis Factor-alpha (TNF-α) and Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) and they make newly recruited cells proliferate and differentiate, i.e. maturing so they can perform specific actions. So the original signal from the macrophage has resulted in the infected area being swamped with new soldiers fighting the invader!