Chemical messengers interact with receptors by forming chemical bonds with them. Because chemical messengers and receptors are three-dimensional objects they have distinct shapes.

The shapes that fit best together allow the most bonds to form. In this way a receptor is able to distinguish between a chemical messenger - the true signal, and a similar compound that fits less well.

We refer to this as specificity. Consequently epinephrine binds with adrenoceptors, histamine binds with histamine receptors and so on.