Any of a large family of proteins, found in all groups of organisms, that are characterized by irreversibly inhibiting serine proteases (i.e. proteases with a serine residue at the active site). Serpins, which can occur both inside and outside cells, are involved in a wide range of metabolic processes, including regulation of complement activation, blood coagulation, tumour suppression, inflammation, and blood vessel formation (angiogenesis). The most abundant serpin in human plasma in α‎₁-antitrypsin, which inhibits trypsin and, most significantly, elastase released by leucocytes. Some serpins lack inhibitory activity, including angiotensinogen, the precursor of angiotensin.