I personally don't recommend giving blood, because blood is the vehicle of the spirit, and when you mix blood, forces from the other individual saturate one's atmosphere. However, it is not necessary to be fanatic: if someone you know who needs blood that only you could provide, it is necessary to do so. This depends upon the judgment of the heart.
Joyful in hope, suffering in tribulation, be thou constant in thy prayer.
Benedictis, qui venit in nomine Domini. Osanna in excelsis.
"Blessed is he who comes in the name of the Lord. Hosanna in the highest!"