A cytokine is ______.

• A. a carbohydrate that provides structural support to the cell wall
• B. a lipid molecule that stores energy in cells
• C. a small protein that acts as a signaling molecule in the immune system
• D. an enzyme that catalyzes DNA replication

Answer: (C)

Verified step by step guidance

1. Understand the term 'cytokine' by breaking it down: 'cyto-' refers to 'cell' and '-kine' refers to 'movement' or 'activity', indicating its role in cellular communication.
2. Recall that cytokines are small proteins secreted by cells, particularly in the immune system, to regulate and mediate immune responses, inflammation, and cell signaling.
3. Eliminate incorrect options: A carbohydrate provides structural support (e.g., cellulose in plant cell walls), which is unrelated to signaling. A lipid stores energy (e.g., triglycerides), which is also unrelated to signaling. An enzyme catalyzes biochemical reactions (e.g., DNA polymerase in DNA replication), which is not the function of cytokines.
4. Identify the correct option: Cytokines are small proteins that act as signaling molecules, particularly in the immune system, to coordinate responses such as inflammation or defense against pathogens.
5. Conclude that the correct answer is: 'a small protein that acts as a signaling molecule in the immune system.'