chemical messenger

Cells cannot speak, but they can hand notes to one another. A chemical messenger is one of those notes — a molecule released by one cell to carry a regulatory instruction to another cell that knows how to read it. Hormones are the most famous members of this family, but they are far from the only ones.

A chemical messenger is any molecule that conveys a signal between cells by binding a specific receptor and triggering a response. The category is broad: it includes hormones (carried in blood), neurotransmitters (released across synapses), paracrine factors (acting on neighbors), cytokines (in the immune system), and gases such as nitric oxide. What unites them is the logic of message, receptor, and response.

Endocrinology focuses on the subset of chemical messengers that act as hormones, but the boundaries are porous — the same molecule, such as noradrenaline, can be a neurotransmitter in one tissue and a circulating hormone in another. A caveat: calling something a chemical messenger says how it works (signaling by binding a receptor), not what chemical class it belongs to.