A G protein-coupled receptor (GPCR) is a type of protein found on the surface of cells that helps transmit signals from outside the cell to the inside. These receptors play a crucial role in various physiological processes, including vision, taste, and the immune response. When a molecule, such as a hormone or neurotransmitter, binds to a GPCR, it activates an internal signaling pathway through associated proteins called G proteins. GPCRs are essential for many bodily functions and are involved in numerous diseases, making them important targets for drug development. There are thousands of different GPCRs in the human body, each responding to specific signals, which allows for a wide range of cellular responses.