Ligand receptors are specialized molecules on the surface of cells that allow certain small molecules to bind to them, inducing a chemical change inside the cell. For simplicity, ligand receptors are usually referred to as receptors, while the small molecules that bind to them are called ligands. Ligands are diverse in size and composition, and there are many classes of receptors specialized to handle them. Receptors can handle ligands ranging from a single ion to a complex protein, depending on the cellular process triggered by activation or inactivation of the receptor.
Specifically, ligands and ligand receptors form a complex when they bind together, and the interactions between them while in that complex spark cellular processes. Receptors are often meant to be specific for one type of ligand, but sometimes other molecules can mimic the required ligand and activate or deactivate the receptor. Certain types of pharmaceuticals take advantage of this and manually activate receptors in this way to fight disease or correct a cellular imbalance. As long as the proper types of interactions occur in the binding complex, a mimic can activate or deactivate a receptor in the same way its usual ligand can.
Receptors can bind to ligands in one of two general patterns, called lock and key or induced fit. The lock and key method of binding is used when a receptor is very rigid and selective about the type of ligand that can bind to it. Only a ligand that fits perfectly together can activate it, like a jigsaw puzzle piece can only go in one place. The induced fit method of binding characterizes receptors that are somewhat flexible, and can modify themselves to bind without being too stringent. In this case, a ligand doesn't have to fit perfectly into the receptor, but the receptor can still trigger an appropriate cellular process.
The interplay of ligands and ligand receptors is an important process in the functioning of many organisms and of the human body. Cells are constantly releasing signal triggering molecules like ligands to tell each other how well the body is functioning. When the ligands bind to receptors, the cells can react to the changes that released the ligands in the first place, and release ligands of their own. The ligands and ligand receptors are in a constant loop giving feedback about cells and the surrounding environment, ensuring that an organism continues to function at maximum capacity.