Here's a defense against bacterial infection that you might not have heard of: "nutritional immunity". It's been known for a long time that bacterial and protozoans have easier access to some vital nutrients than they do to others, and that one particular bottleneck is iron. Iron is a vital component for some key enzymes, and the problem is that bioavailable iron is relatively hard to come by. Its two common oxidation states (+2 and +3) have behaviors that don't help much: iron (II) is very susceptible to oxidation, not least with plain ol' atmospheric oxygen, and the iron (III) compounds tend to have terrible solubility. Iron (III) oxide, known as rust, is one of the most ridiculously insoluble substances you'll ever come across, and that's where a lot of iron tends to end up. Iron and oxygen share another quality: they're simultaneously essential and toxic. Iron species can be a great way to generate oxygen radicals, which are not what you want running around in your cells under ordinary conditions. You see it used biologically for that purpose in enzymes like the CYPs where it's hidden down in an active site, but free iron can be…