Which substances are considered cytokines?

Cytokines are a diverse group of small proteins that are secreted by cells and play a critical role in cell signaling, particularly in the immune response. They are produced by a variety of cells, including immune cells such as lymphocytes, macrophages, and dendritic cells, and they help regulate the balance between humoral and cell-based immune responses. Additionally, cytokines can facilitate communication between cells to enhance or inhibit inflammation and immune activities, influencing processes such as cell proliferation, differentiation, and apoptosis.

The context presented in the other choices does not align with the definition of cytokines. Proteins secreted by the liver encompass a wide range of substances, including plasma proteins such as albumin and clotting factors, which are not specifically categorized as cytokines. Hormones involved in the regulation of blood pressure do not fit the role of cytokines since they are typically classified as endocrine signals rather than immunomodulatory factors. Enzymes that break down bacteria would rather be categorized within the broader function of the immune system related to pathogen clearance, but they do not serve the signaling roles characteristic of cytokines. Thus, the definition outlined in the correct choice captures the essence of what cytokines are and underscores their importance within the immune and inflammatory processes