When unwanted microbes enter your body, your white blood cells rush to the scene to begin fighting them off. But much like a battalion of soldiers, the process by which the cells fight off the invaders needs to be extremely precise, otherwise healthy parts of the body could be damaged by a kind of biological friendly fire. Researchers at the University of Basel in Switzerland (UB), have just uncovered the role of a key enzyme in the process that allows white blood cells to attack with sniper-like precision.

The enzyme is called MPO, or myeloperoxidase, and is actually the compound responsible for giving a green hue to pus at infection sites. When a white blood cell approaches an invading bacterium, it releases hydrogen peroxide (H2O2). MPO converts that substance into HOCL, or hypochloric acid (better known as bleach), which acts like a tiny detonation that wipes out invaders in a radius measuring less than .1 micrometers by burning holes in them.

“Bacteria are helpless against this acid bomb,” explains UB professor Dirk Bumann who co-headed the research. “As hypochloric acid is so highly reactive, the bomb reacts immediately with the closest biomolecules. It is ignited locally and does not spread to the wider surroundings. The bacteria die and the surrounding tissue is spared.”

Basically, the MPO acts as a containment system that ensures that the converted hydrogen peroxide is only released in a small area.

To see just what happens when MPO is absent from the body, the researchers looked at cells from people who don’t have the enzyme because of a genetic effect. In those individuals, the white blood cells still release H2O2 in the face of an invader, but it is never converted. It simply leaks out of the blood cells. This has the result of killing the bacteria, but also damaging the surrounding tissue. “The collateral damage of blood cells and tissues without MPO may cause long-term consequences such as accelerated aging and cancer, but this has not yet been systematically investigated,” added UB’s Nina Khanna, the other investigatory head in the study.

Additionally, the researchers found that when mice lacking MPO were infected with Salmonella, the release of H2O2 also caused tissue damage along with “exacerbated oxidative damage of host lipids and DNA.”

The researchers say the findings could help lead to new treatment strategies to fight bacterial infections through the bolstering of MPO.