A cytokine is a small protein or glycoprotein that is secreted by cells and has various effects on other cells. It plays an important role in cell signaling, particularly in immune responses, hormone regulation, and inflammation. Cytokines are involved in the communication between different types of cells and can either promote or inhibit cell growth, depending on their specific function. They are produced by various cell types including macrophages, lymphocytes, and endothelial cells, among others.