Question: Are we allowed to donate blood to blood banks?
Answer: Where there is a genuine need for blood in the area, for example where lots of accidents are happening and supplies of blood in hospitals are running low, leading to the possibility that an injured person seriously in need of blood may die because of a lack of blood, then giving is allowed. In fact, in severe circumstances we should offer help by donating our blood.
However, where supplies are already at a high level, giving blood would not be allowed. This is because islamically, giving away any part of your body, irregardless of whether it is replenished by the body or not, is not allowed unless there is a real need for it.
(kitabul fatawaa num 2192)