Thymalin is a thymus-derived polypeptide (composed of 5 amino acids) initially isolated from bovine thymus extract. As a synthetic analog of thymic hormones, it plays a crucial role in regulating the immune system, enhancing immune function by promoting the maturation, differentiation, and function of T lymphocytes. Clinically, it is primarily used to treat immunodeficiency states, infectious diseases, and diseases associated with weakened immune function.