What is the definition of a cytokine?

A cytokine is defined specifically as a type of protein that plays a critical role in cell signaling within the immune system. These proteins are produced by various cells and act as messengers to mediate and regulate immunity, inflammation, and hematopoiesis (the formation of blood cells).

The correct choice emphasizes that cytokines influence the immune system's various functions, which include promoting cell communication among immune cells, initiating the immune response, and guiding the differentiation and proliferation of these cells in response to pathogens or injury. They can be pro-inflammatory, stimulating immune responses, or anti-inflammatory, helping to resolve inflammation.

In contrast, while some proteins do enhance the immune response, this does not encompass the full definition of cytokines, as these proteins can also play roles beyond just enhancement. Similarly, describing a cytokine as a type of hormone is misleading, while some cytokines might have hormone-like effects, they are primarily distinguished by their role in immune system signaling. Additionally, carbohydrates are unrelated to the definition of cytokines, as they do not function in the way cytokines do in immune signaling. Thus, the most precise answer aligns with the established understanding of cytokines as proteins that distinctly affect the immune system.