Is donating blood a mitzvah?
Are there contemporary sources?
I am looking for an answer from a posek, rav, etc.
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HaRav Eliezer Waldenberg zt"l has a famous piece in his Tzitz Eliezer (vol 16, siman 23). He notes that it is definitely a mitzvah but one is not obligated (and would in fact be forbidden, if it would cause a decline in health of the donor).
מי שחפץ ברצון עצמו לתרום מדמו ומרגיש שלא יוזק מזה, מדת חסידות יש בזה, ואשרי חלקו מי שיוכל לעמוד בזה
Someone who desires to donate his own blood and feels that he will not be harmed by it, there is an act of piety in this, and fortunate is the portion of one who is able to withstand this.
R Abraham S. Abraham, the author of the Nishmat Avraham compendium of medical halacha, writes (vol 2, p. 346)
Rav [Shlomo Zalman] Auerbach wrote to me that he also permits donating blood to the blood bank in Israel, for the donor performs a mitzvah of saving a Jewish life, even if does so only for the money.
See also this detailed analysis by R Aryeh Lebowitz which concludes similarly.
See Rav Shmuel Halevi Wosner(Vozner) in his Shevet Halevi who says definitively that it is a mitzvah.
See as well Rav Avraham Abraham in his Nishmas Avraham(Vol. 3(Even Haezer), Siman 80, Os 1) who writes that it is a mitzvah to give bone marrow for a bone marrow transplant, he quotes his response with Rav Shlomo Zalman Auerbach who agrees. I assume the same would apply to blood donations which is a significantly less invasive procedure.
The article by Rav Aryeh Levbowitz that Dov quoted above is very helpful.