Any molecule that binds to a receptor — includes both the body’s own hormones and drugs.A receptor may have multiple ligands with different selectivity profiles and different effects. The same receptor can be activated (agonist), blocked (antagonist), partially activated (partial agonist), or even constitutively suppressed (inverse agonist) depending on the ligand and its binding characteristics. Understanding ligand-receptor relationships is foundational to understanding why different compounds in the same receptor family (e.g., different melanocortin agonists) produce different physiological effects.