Peptides are naturally occurring biological molecules. They are short chains of aminoacid monomers linked by peptide (amide) bonds. The covalent chemical bonds are formed when the carboxyl group of one amino acid reacts with the amino group of another. The shortest peptides are dipeptides, consisting of 2 aminoacids joined by a single peptide bond, followed by tripeptides, tetrapeptides, etc.
see Peptide vaccine.