Cytokines

Cytokines are small proteins (approximately 5-20 kDa) that play important roles in cell signaling. Their release influences the behavior of neighboring cells. It is well-known that cytokines are involved in autocrine, paracrine, and endocrine signaling as immune modulating agents. The definitive distinction between cytokines and hormones is still unclear and requires further research.

Cytokines include chemokines, interferons (such as IFN-α, IFN-β, IFN-γ, etc.), interleukins, lymphokines, and tumor necrosis factors, but generally not hormones or growth factors, despite some overlap in terminology. Cytokines are produced by a wide range of cells, including immune cells such as macrophages, T lymphocytes, and mast cells, as well as B lymphocytes, endothelial cells, fibroblasts, and various stromal cells. A given cytokine may be produced by more than one type of cell.

Cytokines, particularly anti-inflammatory cytokines, act through specific receptors and are especially important in the immune system. They modulate the balance between humoral and cell-based immune responses and regulate the maturation, growth, and responsiveness of particular cell populations. Some cytokines can inhibit or enhance the actions of other cytokines in complex ways.

Cytokines are different from hormones, which are also important cell signaling molecules, in several key aspects. Hormones typically circulate in more stable concentrations and are usually produced by specific types of cells.

Cytokines play essential roles in both health and disease. They are particularly important in host responses to infection, immune responses, inflammatory responses (both acute and chronic), trauma, sepsis, cancer, and reproduction.