The peptides studied in this field are largely signalling molecules. They are released by one cell, travel a short or long distance, and bind a receptor on another cell — carrying an instruction rather than performing work themselves. Insulin, glucagon and the incretins are the textbook examples, and the literature on peptide hormone receptors is built on them.
A peptide does not act on the body directly. It acts on a receptor, and the receptor determines what happens next. So the same molecule can produce entirely different effects in different tissues, depending on which receptors those tissues carry and what those receptors are wired to inside the cell.
