Receptor: This is considered to be the point of interaction between a drug and a biological system. It can include a binding component for drug recognition as well as signaling (transduction, amplification) mechanisms. In most cases drugs act via interaction with specific macromolecules in the organism. This interaction leads to the chain of events that culminates in the drugs observed effects. Receptors are classified according structure (e.g., ligand gated ion channel, G-protein linked) and the molecules which activate them (e.g., adrenergic, cholinergic).