Why does your body not attack metal plates and screws after surgeries?
In: Biology
It does. Or at least, it *tries*. Those metal parts are made out of metals that are particularly difficult to attack because they are very nonreactive.
Almost all metals react with oxygen. With iron, we call this *rust*. When iron rusts, the resulting iron oxide expands and doesn’t stick well to the iron underneath, so it flakes off and the iron underneath is exposed to oxygen and then *it* rusts. The oxides of a lot of metals, though, sticks quite well to the metal underneath. Once the metal is oxidized, it’s *really* hard for anything else to get to it. Oxygen really does not like to share. So, that oxidized layer forms a kind of armor, where the metal is “pre-reacted” and won’t react well to anything else.
Titanium is one such metal. Titanium oxide is tough stuff. Add a little of some other metals and you make something that your body just doesn’t have any tools to break apart.
Being non-reactive also helps hide it from your immune system. Your immune system doesn’t have eyes – it can only “see” what reacts to the antigen receptors on the outside of immune cells. Immune cells learn that certain antigens indicate *you* and shouldn’t be attacked, and antigens that *aren’t you* are foreign and should be attacked. All the bacteria living in your body have adapted to hide their bacterial antigens and give off fake “I’m part of you, don’t attack me” antigens. But something that doesn’t react *at all* doesn’t have antigens. Your immune cells just bounce off of it and never really notice that it’s there.
Since it “isn’t there” your immune system doesn’t try very hard to get rid of it, and since your immune system doesn’t have any tools that can get through that protective oxide layer anyway, the metal is fine.
It *is* possible, though, for a body to notice and try to reject the metal part. That usually doesn’t do anything to the metal, but the inflammation can be dangerous for the cells around the metal part and cause problems for your body. In that case, anti-rejection drugs that suppress your immune system will *hopefully* get your body to stop attacking until it gets used to the part being there and “forgets” to attack it. Or, they have to try a different alloy.
This is also why your body doesn’t attack medical plastics. The plastic is very nonreactive. And, it’s why bullets are usually left inside the patient. Elemental lead is pretty nonreactive so although your body will try to attack it, which will put some lead into your blood to spread through your body, it’s such a slow process that it isn’t going to hurt you. The surgery to remove the bullet will cause more damage. (Unless the bullet is like, right next to your heart and you really need to get it out.)
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