A second messenger is a small intracellular molecule or ion whose concentration, localization, or chemical state changes after receptor activation and which then regulates downstream proteins.

The extracellular ligand is often called the first messenger. It may never enter the cell. Instead, receptor activation changes production, release, or removal of a second messenger inside the cell.

Common second messengers include

• cyclic AMP (cAMP);
• calcium ions (Ca$^{2+}$);
• inositol 1,4,5-trisphosphate (IP$_3$);
• diacylglycerol (DAG).

Why use a second messenger?

A receptor is localized to a particular molecular site, while a second messenger can distribute information to many downstream targets.

The architecture is

extracellular ligand
      ↓ receptor
second-messenger production or release
      ↓
many intracellular targets

This can amplify a signal because one activated receptor or enzyme can generate many second-messenger molecules.

Spatial localization still matters

Second messengers do not necessarily become uniform throughout the cell. Their production and removal can be localized, and diffusion can be limited by binding, degradation, membrane boundaries, or active pumping.

For example, cytosolic Ca$^{2+}$ can rise briefly near an open channel and then fall as pumps and binding proteins remove free Ca$^{2+}$.

A messenger must also be reset

A useful signaling intermediate must be reversible. cAMP can be enzymatically degraded; Ca$^{2+}$ can be pumped out of the cytosol or into intracellular stores; IP$_3$ can be metabolized; DAG can be converted into other lipids.

A second messenger is therefore not merely a molecule found in a pathway. Its defining role is to carry receptor-generated information inside the cell in a controllable and reversible form.