Cytokines and Chemokines

Cytokines and chemokines are small biologically active molecules that can act locally or at a distance. Cytokines mediate cell-to-cell communication, regulate both the initiation and maintenance of immune responses, often define the function of T lymphocytes, and can induce intrinsic cellular defenses. Cytokine families, most with several members, include TNF, IL-1, IFN-γ, IL-2, IL-4, IL-6, IL-7, IL-10, IL-12, and IL-17. Cytokine receptors have a modular design and individual chains may participate in formation of the receptor for more than one cytokine. Chemokines attract specific, but overlapping, populations of inflammatory cells and regulate trafficking of leukocytes during immune responses and infiltration of leukocytes into infected tissues. Chemokines are divided into four families based on spacing of the first two of four conserved disulfide-forming cysteine residues. The CXC or β chemokines are primarily chemotactic for neutrophils. The CC or α chemokines are primarily chemotactic for monocytes, lymphocytes, or eosinophils. Chemokine receptors are seven-transmembrane G-protein-coupled molecules that form distinct families of CXC and CC receptors. Each receptor can bind multiple chemokines within its class and each chemokine can bind multiple receptors. A number of viruses, particularly in the herpesvirus and poxvirus families, encode inhibitory homologs of cytokines, chemokines, or their receptors, attesting to their importance for antiviral immunity.