The primary means by which cell-to-cell communication as well as homeostasis in multicellular organisms occurs, involving protein receptors by receiving cells.
Chemical signaling can involve small molecules (ligands) or large molecules (cell-surface signaling proteins). This signaling can be received either on the surface of cells by receptor proteins or, more rarely, within the interior of cells but also by receptor proteins (an example of within-cell reception of signals is of steroid hormones).
Signals also can be intentionally provided, such as is the case of hormones, or instead can be present for reasons that are not specifically for the purpose of providing a signal (e.g., carbon dioxide levels in blood).
Cell-to-cell communication can be considered to be a product of intentional chemical signals whereas the maintenance by organisms of homeostasis can be viewed as ongoing responses to "unintiontional" chemical as well as non-chemical signals. As a complication, very often intentional chemical signals, such as hormones, may be released in response to unintentional signals, such as glucose concentrations in blood following a meal which stimulates the release of insulin.
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