Jehovas Witnesses. Why no blood transfusion?

I believe they refuse transfusion because it is considered unclean. My bro is JW (yuk) and when he had surgery, he was put on like a byp and his own blood was recirculated and put back in him. They will allow their own blood to be put back in their bodies. And-He and his wife eat meat, so does my sis who is also JW. NO I AM NOT A JW, THANK GOD!