Mitochondria are the cell organelles that produce hydrogen peroxide as a byproduct of respiration. Hydrogen peroxide can be used by the human body to fight infection, but its production may also sometimes cause harmful effects on health.
Hydrogen peroxide produced within cells in response to infection causes the cells to attract and bind to leukocytes (white blood cells), which then directly attack the organisms causing the infection. Because hydrogen peroxide appears to play a role in inflammatory responses, which can be helpful to the immune system, it may also be implicated in some health problems related to inflammation.
Mitochondria are also involved in normal cell death and with the breakdown of sugar within cells, which leads to the release of adenosine triphosphate, the source of cellular energy.