A large class of proteins, including interferons. which are secreted by certain cells of the immune system and have an effect on other cells. Their function is to communicate between cells to trigger and regulate or mediate the protective defences of the immune system that helps to destroy pathogens.
These immune system processes lead to inflammation (e.g. redness, swelling, soreness) which indicates that the system is working as it should. There are times however when cytokines go into overdrive leading to a cytokine storm that puts normal body function at risk.