What are cytokines best described as?

Cytokines are best described as small signaling molecules made and secreted by cells. They play a crucial role in cell communication, especially in the immune system where they help modulate the immune response. Cytokines can be produced by various cell types and have a wide range of functions, including regulating inflammation, cell growth, and differentiation. Their ability to act on distant cells or the cells that secrete them is fundamental to their role in cellular signaling pathways.

The other descriptions do not accurately capture the nature of cytokines; for instance, while cytokines do influence cellular behavior, they are primarily categorized as small molecules rather than large ones. Additionally, cytokines are not responsible for cellular respiration nor are they components of the extracellular matrix, which further distinguishes their specific biological function within organisms.