Birdie; I'm not looking for argument but just want to mediate, as I said earlier he (and they) do have a point. Have you seen;
http://www.mayoclinic.org/tests-procedures/blood-transfusion/basics/risks/prc-20021256
Blood is an extraordinary substance containing millions of ingredients, some testable - hepatitis etc. and some not. I live, walk and play golf in an area of high risk from tic bites, and so have injections against contracting Lymes disease, a good thing perhaps, but antibodies would be in any blood I donated. Your blood may contain other things you have been inoculated against. Blood groups were only discovered about a 100 years ago long-term effects of mixing blood could lead to a lot of unknown complications, though having said that it can and does save lives every day.