Why does the body attack a transplanted organ?

A body attacks a transplanted organ because it does not recognize it as being made of self cells. It knows this because the new organ has antigens, a form of proteins that coat the organ. When a body's immune system recognizes these antigens and they are not similar enough to the body's own antigen's, it attacks to try and keep itself safe and healthy.

Explanation:

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds