Real receptors are receptors for what type of ligands?

• A. Endogenous regulatory ligands.
• B. Pheromones.
• C. Exogenous drugs only.
• D. Ion channels.

Answer: (C)

Receptors are proteins that recognize and bind the body's own signaling molecules—endogenous regulatory ligands like neurotransmitters and hormones. This binding initiates the physiological responses that coordinate functions across tissues and systems. Drugs and other external substances can also bind to these receptors, altering signaling, but they are external ligands, not the natural partners those receptors evolved to recognize. Pheromones act on sensory systems, and ion channels describe a class of proteins that mediate ion flow rather than being ligands themselves. The core idea is that receptors are designed to respond to endogenous regulatory ligands.