Hormones are chemical messengers secreted into the blood or extracellular fluid by one cell that affect the functioning of other cells. Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone.
Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone. In very basic terms, binding of a hormone to a receptor triggers a cascade of reactions within the cell that affects function.
A traditional part of the definition of hormones described them as being secreted into blood and affecting cells at distant sites. However, many of the hormones known to act in that manner have also been shown to affect neighboring cells or even have effects on the same cells that secreted the hormone. Nonetheless, it is useful to be able to describe how the signal is distributed for a particular hormonal pathway, and three actions are defined:
- Endocrine – the hormone is distributed in blood and binds to distant target cells.
- Paracrine – the hormone acts locally by diffusing from its source to target cells in the neighborhood.
- Autocrine – the hormone acts on the same cell that produced it.
Two types of molecules bind to the hormone binding sites: Agonists are the hormones themselves. They bind the receptor and trigger all the post-receptor events that lead to a biologic effect. Antagonists are molecules that bind the receptor thereby blocking the agonist, preventing the intracellular signaling system.
