Amino acids link together through peptide bonds to form a chain. When the chain runs to roughly fifty amino acids or fewer, it is conventionally called a peptide; longer chains are called proteins. That cut-off is a naming convention rather than a sharp chemical line, and different sources place it in slightly different spots.
That size is the whole reason researchers find the class interesting. A peptide is big enough to carry the specific three-dimensional shape a receptor recognises, yet small enough to be built to an exact sequence in a laboratory. Reviews of therapeutic peptides describe this combination — the pinpoint targeting of a large biologic drug together with the easier manufacturing of a small molecule — as the field’s defining characteristic.
