Within an organism it is critical that signals between cells are very specific.

For example, if ligand A is meant to activate immune system cells to reproduce in response to an infection, it should not also cause other cells to grow as if they had received a growth hormone.

When a ligand is released, what prevents all of the cells in the body from being affected?

The ligand has only one receptor that it is able to bind to

Explanation:

The shape of the ligand and its corresponding receptor allow there to be specificity. For example, receptors for ligand A would be found on the target immune system cells and not on cardiac muscle cells or skin epithelial cells. Therfore, even though the cardiac muscle cells or the skin epithelial cells would be exposed to ligand A, they would not be able to bind to it and therefore could not react because of the specificity of the receptor of ligand A at that moment