A person received a letter from a hospital, apparently in Eretz Yisroel, that his bone marrow was needed to save a dying patient with whom he was a rare match. However, the hospital refused to disclose who the patient is and no details were revealed. The person asked Rav Moshe Shternbuch, Rosh Av Bais Din of the Eidah Hachareidis, if he was obligated to undergo the transplant procedure under these circumstances, considering that the donation may be to a person whose life he is not obligated to save.
Rav Shternbuch (Teshuvos Vehanhagos 5:387) paskened that halachicaly there is no obligation to believe the hospital. In the case of pikuach nefesh, the poskim require that the halachos only apply if the case is clear. In this case, there is no way to determine if the transplant will help and what the chances of success are. It is also possible that there are other available donors and they don’t necessarily need him.
Moreover, mei’ikkar hadin, a person is not obligated to give a part of himself to save his friend. For example, one is not obligated to donate a kidney to save someone’s life, even if there is no danger involved.
With regard to bone marrow, however, it is unclear in the medical world if it is like a kidney transplant or a blood donation which you completely recover from without any change in the body.
Although halachicaly there is no obligation, says Rav Shternbuch, if you do the procedure lesheim Shomayim, it is permissible and you have a great mitzvah. Even if it will cause you to miss a few days of tefillah betzibuur, for example, and even if it will go to children of mixed marriages, still,l there is a good chance that it will go to a Jew, many of whom are tinokos shenishbuh, and you have done a great mitzvah.