<P> Iron is also potentially toxic . Its ability to donate and accept electrons means that it can catalyze the conversion of hydrogen peroxide into free radicals . Free radicals can cause damage to a wide variety of cellular structures, and ultimately kill the cell . </P> <P> Iron bound to proteins or cofactors such as heme is safe . Also, there are virtually no truly free iron ions in the cell, since they readily form complexes with organic molecules . However, some of the intracellular iron is bound to low - affinity complexes, and is termed labile iron or "free" iron . Iron in such complexes can cause damage as described above . </P> <P> To prevent that kind of damage, all life forms that use iron bind the iron atoms to proteins . This binding allows cells to benefit from iron while also limiting its ability to do harm . Typical intracellular labile iron concentrations in bacteria are 10 - 20 micromolar, though they can be 10-fold higher in anaerobic environment, where free radicals and reactive oxygen species are scarcer . In mammalian cells, intracellular labile iron concentrations are typically smaller than 1 micromolar, less than 5 percent of total cellular iron . </P> <P> In response to a systemic bacterial infection, the immune system initiates a process known as iron withholding . If bacteria are to survive, then they must obtain iron from their environment . Disease - causing bacteria do this in many ways, including releasing iron - binding molecules called siderophores and then reabsorbing them to recover iron, or scavenging iron from hemoglobin and transferrin . The harder they have to work to get iron, the greater a metabolic price they must pay . That means that iron - deprived bacteria reproduce more slowly . So our control of iron levels appears to be an important defense against most bacterial infections; there are some exceptions however . TB causing bacterium can reside within macrophages which are an iron rich environment and Borrelia burgdorferi utilises manganese in place of iron . People with increased amounts of iron, like people with hemochromatosis, are more susceptible to some bacterial infection . </P>

Where does iron get stored in the body