any of various protein molecules secreted by cells of the immune system that serve to regulate the immune system
"When my doctor explained why I needed blood work after developing a persistent fever, she mentioned that elevated levels of certain cytokines were indicating an active infection fighting within my body."
Any of various small regulatory proteins that regulate the cells of the immune system.
"When the virus invaded, her body flooded its tissues with cytokines to rally the immune cells against the infection."