answer:It will kill you dead. Panic now! Ok, seriously. As someone who has donated blood before, many times, it’s not a big deal. If you have an underlying condition that you don’t know about, they might find it when they test your blood, so you will know about it. Losing a pint of blood or whatever won’t kill you or harm you, unless you’re missing much more already! Plus they usually provide you with a meal or something after you donate.