system
Given the following post, respond as if you're commenting on Reddit:
human
In what way does a body accept a new donated organ? Is it related to blood type? What causes it to be rejected?
gpt
It is not related to blood type. Every human cell expresses a type of proteins called major histocompatability proteins (MHC proteins) that serve as a marker for "self cells". The sequence and "shape" of these proteins varies greatly between people. Your white blood cells will destroy any cells or tissues expressing MHC proteins different from your own. While developing, the immune system destroys any white blood cells that recognize your set of MHC proteins, leaving only white blood cells that recognize different MHC sets. 

For a successful organ transplant, the donor and recipient must have pretty similar MHC proteins. Siblings are often the best for this, as, often, even parents' MHC proteins are too different. MHC shape and sequence are essentially random, so if no family member is available random people are screened to see if they are an MHC match.