system
Given the following post, respond as if you're commenting on Reddit:
human
Why are cardiac valves from pigs not rejected by human bodies?

The problem with the body rejecting organs/tissues is huge and mostly entails the usage of pharmaceuticals preventing the rejection. Why can be transplant cardiac valves from pigs without having a rejection (in most cases)?
gpt
Roughly speaking, all tissue in your body has cells (which are alive) and other things (which aren't). Some tissue (like muscles) has a lot of cells and a little of other things. Some tissue (like cartilage) has very few cells and is mostly made of other things.

Immune reactions primarily target living tissue. 

Heart valves are made of is almost entirely non-living tissue, and after being harvested from pigs, those valves are treated with chemicals to kill off any residual pig cells. As a result, when they are implanted, they usually do not cause immune reactions because there are no cells within them to cause an immune reaction.

This is similar to receiving ligament grafts from deceased human donors.