Why does the body attack a transplanted organ?

A transplanted organ is viewed by our bodies immune system as a foreign object, much like when we get sick. Our body is just trying to protect itself. For this reason people that have a transplant have to take immunosuppression medication for the rest of their lives.

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an active site is a region that is present on an enzyme where the reactants or the substrate molecules we can say bind and experience a chemical reaction. the substrate is basically a reactant whose concentration is changing and that is converting into a product after binding at the active site of an enzyme