Cells communicate by releasing or presenting signals that are interpreted by other cells. A receptor is a molecule that binds or detects a signal and couples detection to a response. A target cell is a cell that has the molecular machinery needed to respond to a particular signal.

Several recurring signaling geometries are useful.

• Autocrine signaling: a cell responds to a signal that it releases itself or that is released by nearby cells of the same type.
• Paracrine signaling: a signal acts over a short distance on nearby cells.
• Endocrine signaling: a secreted signal, called a hormone in this context, is transported over long distances through body fluids to distant targets.
• Contact-dependent signaling: a membrane-bound signal on one cell binds a receptor on an adjacent cell, so direct contact is required.
• Direct cytoplasmic communication: channels connecting neighboring cells can allow selected small molecules or ions to pass directly from one cytoplasm to another.

Distance does not determine specificity by itself

An endocrine signal can circulate past many cell types, yet only cells carrying an appropriate receptor respond strongly. Conversely, two neighboring cells can react differently to the same ligand because they express different receptors or different downstream signaling machinery.

Thus

signal present + compatible receptor + competent downstream machinery
                         ↓
                    cellular response

The same ligand can produce different responses

Receptor identity and cell state matter. A ligand can activate one pathway in one cell type and a different pathway in another if the receptor subtype or downstream proteins differ.

Cell signaling therefore has two distinct layers of selectivity:

1. delivery: where the signaling molecule can reach;
2. interpretation: which cells possess the molecular machinery to respond.

This distinction is central to understanding how a multicellular organism can coordinate many cell types using a limited set of signaling molecules.