A receptor agonist is a substance that binds to a specific receptor in the body and activates it, mimicking the action of a natural substance. This interaction can trigger various biological responses, influencing processes such as mood, pain perception, and immune function. Common examples include certain medications that target neurotransmitter receptors in the brain. These agonists can be naturally occurring, like hormones or neurotransmitters, or synthetic compounds designed for therapeutic use. For instance, morphine acts as an agonist at opioid receptors, providing pain relief by activating these receptors in the nervous system.