system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5; Metal implants not being rejected by body?

Why can a body accept metal appliances without trying to expel it? Like a bullet lodged or hardware used to repair a broken bone. Wouldn’t our body try to push the foreign body out, like it does a splinter?
gpt
Splinters get pushed out because they are stuck on the surface layers of our skin. The upper layer of the skin is made from dead skin cells that get pushed up from the living skin cells dividing to replenish the upper layer. Other forces could be due to muscles flexing to push foreign particles in a direction.

When the implants are stuck into our bones they really don't have anywhere to go.

With that said there are many ways for our body to "reject" an implant. Focusing on bone implants, the metal can corrode the surrounding bone, making the connection between implant and bone weak until it fails. The metal can release ions or other chemical reactions that actively harm the nearby cells or body.

 The body is also a pretty harsh environment for metal since it's covered in mostly water. This can corrode the metal in the implant and cause it to fail. Corroding the implant can also cause the implant surface to roughen so that moving it in the body hurts the surrounding cells, causing inflammation. You also want a metal where the body doesn't recognize it as an enemy so the immune response doesn't attack the metal directly.

In general implants are quite difficult to both design and approve for use since there are many ways for it be rejected over the lifetime of the implant.