Q. What is the Jewish attitude to blood donations where there definitely isn’t any “known” recipient?
A. The issue of a “known” recipient – in Hebrew, choleh lefanenu, literally “a patient who is before us” – arises with, for example, an organ bank into which, theoretically, human organs could be deposited for use at some future time when needed.
Being a blood donor does not raise this problem at all. Subject to obvious safeguards, removing blood does not harm the donor, and the blood can normally not be immediately transfused into a “known” patient.
Giving blood, as Lord Jakobovits point out in his “Jewish Medical Ethics”, 1975 edition, page 285, is an almost literal fulfilment of the precept, “Do not stand idly by the blood of your neighbour” (Lev. 19:16), which the sages interpret as requiring a person to act when another’s life is at risk.