The following texts can be found in the Holy Scriptures (NWT):
New World Translation of the WTS
These four texts, almost all from the Old Testament (or, as the WTS calls it: from the Hebrew Scriptures), only the last two from the New Testament (in WTS jargon: Christian Greek Scriptures) are used by the Watchtower Society to forbid blood transfusions to its followers. "If the doctor forbids alcohol to his patient, it does not mean that he may take it intravenously instead of orally," is argued.
Question: Does this also apply to blood transfusions?
Well, taking blood to eat is and remains forbidden. That is clear from the HS. It is quite simply seen, so written, yes, also in the New Testament.
God Yahweh recognizes no good in it when people eat the soul of animals. For whatever reason, although people do not always see things the same way. But, that doesn't play a big role either, because he is "the boss" and can switch in this regard and rule as he wants. And all those who believe in him know that his switching and acting is never something bad for man.
It is a different matter that not all people, not even those who believe in him, keep his commandments and therefore still like to eat blood sausage. But let's stick to the point where WTS derives from not being allowed to take blood transfusions.
Since the thought (actually: the commandment) not to be allowed to eat blood originally derives from the AT, we can gladly stay with it.
In the same OT, it was strictly forbidden to eat the shewbread exhibited in God's tent for Yahweh.
In the first book of Samuel, chapter 21, we find the story of how David went to a priest named Ahimelech to ask for bread against hunger. But since there were no "normal" loaves of bread, but only shewbread, the priest gave them to David at his request, despite the prohibition. Why? Because an emergency so required. This matter is confirmed by the Lord Jesus himself - to be found in Luke's Gospel chapter 6.
Or: The Passover was bound to a certain date, in memory of the exodus from Egypt. It was a fixed date, not a date that could easily be moved. Nevertheless, Hezekiah, king of the two tribes of Judah, did exactly that. Why? Again for the same reason: Because a special situation required this (Second Book Chronica, Chapter 30).
At a much later time Jesus was rebuked because his disciples did not wash their hands before eating. Likewise, he himself was rebuked for healing the sick on the Sabbath and for picking ears of corn from his disciples. Everything can be read in the New Testament. What did Jesus, who was the fulfiller of the law (Matthew's gospel chapter 5, verse 17), answer? He said: The law was made for man and not the other way round (Mark's gospel chapter 2, verses 23-28).
Question: What can we conclude from these texts on the question of whether blood may be used to save human lives?
So the answer is clear: blood can and may be used in an emergency!
If, of course, someone rejects this anyway, it remains a pure decision of conscience of an individual, which must absolutely be respected, no question! But if someone has another insight, namely that he may take blood during an operation, this must be respected just as much and must not lead to a condemnation, i.e. an exclusion (corresponding to an excommunication in the Roman Catholic Church) of a person from his church/faith community!
An interesting clue yet:
In an official inquiry to the Rabbinate Munich (via "Umweg" Augsburg), the question of how today's Jews see this was answered in exactly the same way. Blood transfusions are not rejected (at least by most) Jews, exactly for the reasons mentioned here (exception: New Testament justification - of course).