Cytokines are a large and diverse family of polypeptide regulators, signaling proteins and glycoproteins, which- like hormones and neurotransmittersare critical to the development and functioning of both innate and adaptive immune response, as well as to other body systems. While hormones are secreted from specific organs to the blood and neurotransmitters are related to neural activity, cytokines are more of a diverse category or class of compounds in terms of origin and function.
These signaling molecules, historically known as immodulating agents (e.g. interferons, interleukins) like neurotransmitters, hormones and growth factors, function as mediators, directly and indirectly, in cellular communication by triggering specific cellular reactions or responses in target tissues and organs throughout many areas of the body, or to distinct, specific cells. Neurotransmitters act in a similar fashion when released at an axon terminal of a neuron that eventually locks on to a specific receptor site or target impacting another neuron, muscle or gland cell. Growth factors like cytokines are secreted by a variety of cells and bind to or lock on to specific high-affinity cell-surface target receptors, thereby eliciting similar or more specific biochemical changes through a cascade of events. Whereas the effects of cytokines include autocrine, endocrine and paracrine activity relative to target organ or cell-receptor site, growth factors not only elicit a cascade of events by participating in the regulation of cell growth and differentiation, but also by promoting replication and similar complex processes.
The cytokine family includes a variety of colony and growth-stimulating factors, interferons, interleukins, chemokines and other molecules that exhibit pleiotropic effects and considerable redundancy. They may act on the cellular level such as in cell differentiation, through tissue development, homeostasis, and in certain developmental processes, during embryogenesis. Due to the fact that they are characterized by such general effects and hence are often difficult to objectively characterize, such aforementioned distinctions, allowing for exceptions, are for the most part null and void. Nevertheless, this much is known about them. These distinct effects are multi-fold and depend upon, inter alia:
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