Myeloperoxidase (MPO) is a human enzyme in the azurophilic granules of neutrophils and in the lysosomes of monocytes. Its major role is to aid in microbial killing. Although MPO received little clinical attention until 1966, the enzyme was first isolated in 1941, and deficiency of MPO was first described in 1954. Some patients with MPO deficiency have impaired microbial killing, but most are asymptomatic.
The condition was initially believed to be very rare with only 15 cases reported before the 1970s. However, modern laboratory techniques have allowed researchers to discover that MPO deficiency is actually more common than previously described but without clinical relevance.