There is really no need to get angry or defensive when it comes to the JW doctrine on blood transfusions.
There are hundredths of different Christian denominations, and each one of them has its own doctrine based on their individual interpretation of the Bible. The JW, however, have been wrong several times when it comes to Bible interpretation. Its founder, Charles Taze Russell, for example, prophesied for many years that the world was going to end in 1914. Obviously he was wrong! In my personal opinion, there is no need to argue the Biblical principle on any of the JW doctrines. Instead, is better to expose the fact that if the JW have been wrong in their Bible interpretations, maybe they're Biblically wrong in their doctrine on blood transfusion.