Ligands are chemical messengers released by cells to transmit signals either to themselves or to other nearby or distant cells. Ligand receptors or cellular receptors are protein molecules found embedded in the cell membrane on the surface of the cell or in the cytoplasm or nucleus within the cell. 

Although there are several different forms of ligands and ligand receptors, they all have one thing in common – they are present in closely matched pairs, with each receptor being able to recognize only one or a few specific ligands and each ligand able to bind to just one or a few target receptors. Binding of a ligand to a receptor on a target cell, changes the receptor’s shape or activity, allowing it to send a signal or directly produce a change inside the cell. 

• Small, hydrophobic ligands, such as steroid hormones in humans, are capable of traversing the cell membrane, binding to intracellular receptors in the nucleus or cytoplasm. 
• Larger, water-soluble ligands are charged and unable to cross the cell membrane. They stay on the outer surface of the cell and bind to the cell-surface reporters.