Myeloperoxidase (MPO) is an iron-containing human enzyme that helps neutrophils and monocytes (types of white blood cells) destroy foreign substances like bacteria, fungi, and viruses that enter the body.
Myeloperoxidase deficiency, a disorder that occurs when individuals have abnormally low or nonexistent levels of the enzyme, was first described in 1954. Some patients who have MPO deficiency are unable to destroy substances that enter the body as efficiently as individuals who have the enzyme. However, most patients are asymptomatic (experience no symptoms) and the condition is usually undiagnosed.
In most cases the condition is inherited. When the condition is caused by other medical conditions, such as anemia, thrombotic diseases, or diabetes mellitus, it is called acquired MPO deficiency. Most acquired cases are temporary and resolve once the underlying cause is treated.
In general, most individuals with partial or complete MPO deficiency have no increased frequency of infections. This is because MPO-deficient neutrophils are able to phagocytose (engulf) most microbes normally.
However, the neutrophils' ability to actually kill the bacteria typically occurs at a slower rate. For instance, when bacteria like Staphylococcus aureus or Escherichia coli enter the body, it takes the neutrophils longer than normal to kill the bacteria. This indicates that the MPO-deficient cells use a different and slower method of killing the microbes.
The MPO-deficient neutrophils are completely unable to kill certain fungi and in vitro (test tube) studies have shown that Candida albicans, Candida krusei, Candida stellatoidea, and Candida tropicalis cannot be killed by MPO-deficient neutrophils.
Severe infections occur in fewer than five percent of patients with MPO deficiency. In general, infections only occur in patients who also have diabetes mellitus. The reason for this remains unknown.
Since most cases are undiagnosed, researchers initially thought that the condition was very rare. Only 15 cases reported worldwide before 1970. Today, technological advances in sciences have led researchers to believe the condition is more prevalent, with incidence rates from studies ranging from one case per 1,400 to 2,000 people in the United States.
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