What characterizes an endogenous ligand?

An endogenous ligand is characterized as a naturally occurring substance in the body. These ligands are produced by the body and interact with receptors to elicit physiological responses. Examples include neurotransmitters, hormones, and peptides that modulate various biological processes like mood, growth, and metabolism. By binding to specific receptors, these endogenous ligands play critical roles in maintaining homeostasis and facilitating communication within the body's systems.

The other options do not align with the definition of an endogenous ligand. For instance, a ligand made outside the body refers to exogenous ligands, which are typically drugs or substances introduced from external sources. A ligand that acts as a full antagonist describes a substance that binds to receptors but does not activate them, which does not pertain to the classification of an endogenous ligand. Similarly, a liganded drug designed for specific treatments refers to synthetic or modified compounds used therapeutically, rather than naturally occurring substances that are classified as endogenous.