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I'm trying to figure out the difference between hormone, cytokine and protein hormone. It's clear to me that all three are biological messengers, but there seems to some ambiguity and overlap between terms and I'm trying to understand the nuances.

In brief:

  • What do each of these three terms mean and how are they different?
  • Do any of the three terms represent a superset of the others? e.g are all cytokines, hormones? Are protein hormones just a subset of hormones?
  • Are the terms context specific at all? For example, cytokines are referenced in the context of immunology below, but are they actually fundamentally different from hormones?
    • If there is context-dependence, under which contexts should each word be used?
    • Are there any canonical examples of each? e.g Prolactin is given as an example of a protein hormone below, how about for cytokines and ordinary hormones?

In June 1905, Ernest Starling, a professor of physiology at University College London, UK, first used the word 'hormone' in one of four Croonian Lectures—'On the chemical correlation of the functions of the body'—delivered at the Royal College of Physicians in London. Starling defined the word, derived from the Greek meaning 'to arouse or excite', as “the chemical messengers which speeding from cell to cell along the blood stream, may coordinate the activities and growth of different parts of the body” [1]

Cytokines and chemoattractant cytokines known as chemokines are highly localized soluble signaling proteins produced by many cells of the immune system (neutrophils, monocytes, macrophages, B-cells, and T-cells) to regulate immune responses [2]

A great deal of evidence has accumulated and confirmed that hormones secreted by the neuroendocrine system play an important role in communication and regulation of the cells of the immune system. Among protein hormones, this has been most clearly documented for prolactin (PRL), growth hormone (GH), and insulin-like growth factor-1 (IGF-I), but significant influences on immunity by thyroid stimulating hormone (TSH) have also been demonstrated. [3]

References

[1]: Tata JR. One hundred years of hormones. EMBO Reports. 2005;6(6):490-496. doi:10.1038/sj.embor.7400444.

[2]: Stenken JA, Poschenrieder AJ. Bioanalytical Chemistry of Cytokines-A Review. Analytica chimica acta. 2015;853:95-115. doi:10.1016/j.aca.2014.10.009.

[3]: Kelley KW, Weigent DA, Kooijman R. Protein Hormones and Immunity. Brain, behavior, and immunity. 2007;21(4):384-392. doi:10.1016/j.bbi.2006.11.010.

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Parts of the answer are in the text that you provide yourself. But I shall try to add where i can.

What do each of these three terms [hormone, cytokine and protein hormone] mean and how are they different?

Both cytokines and hormones are a class of signalling molecules that are secreted by cells.

Cytokines are a group of small protein that have a fundamental role in the immune system. They are typically short lived and typically have local effects and are specific cytokines are often produced by multiple cell types.

For example they cause local cells to generate heat and cause blood vessels to become wider and more permeable to blood cells. parham (2015) Some cytokines can act over longer distances, IL-6 for example cause local inflammation and regulates fever by targeting the hypothalamus causing it to increase body temperature overall. For more information check this link.

Hormones are secreted into the blood by dedicated endocrine cells so the can regulate other cells across the body.(alberts, 2008)

Do any of the three terms represent a superset of the others?

Protein hormones are a subset of hormones. An other important subset are steroid hormones.

It can be argued that cytokines function as hormones in some situations. For example: in the case of an infection high concentrations of cytokines can regulate body temperature causing fever. IL-1α, IL-1β and IL-6 are good examples here

Are the terms context specific at all?

In some situations it can be argued that cytokines act as hormones. But the feature that groups them is that the are an actor in the immune response. This wiki discusses some differences.

Classical hormones are produced by a gland and excreted in the blood. This is not context depended.

Are cytokines actually fundamentally different from hormones?

All Cytokines are proteins and don't act fundamentally different from protein hormones. Both have to act through membrane bound receptors since proteins can not pass the cell membrane on their own. Steroid Hormones can pass the membrane and typically have intracellular receptors.

Are there any canonical examples of each?

Interferons and Interleukins are a large classes of cytokines. IFN-γ and IL-6 a well known examples. Androgens are steroid Hormones . Testosterone is one of them.

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  • $\begingroup$ "The difference is that cytokines act locally" - as you point out elsewhere in your answer, some cytokines act systemically rather than locally. "are produced by many cell types" - this isn't really much of a difference with hormones, since hormones are also produced by a variety of cell types. $\endgroup$ – Bryan Krause Oct 17 '18 at 22:42
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    $\begingroup$ You are right. I could have formulated that both sentence more precise. By "are produced by many cell types" I meant that specific cytokines are typically secreted by multiple different cell types. For example: IFNγ is secreted by T helper cells , cytotoxic T cells , macrophages, mucosal epithelial cells and natural killer cells. Different hormones are indeed secreted by different cells, but most hormones are secreted by one cell type. for example: insulin is onl secreted by Beta cells. I will rewrite my answer to be more precise tomorrow. $\endgroup$ – Jonas Oct 18 '18 at 0:18

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