I have been confused over the fact that the Society does allow some parts of blood. But tells us we can't store our own blood before an operation. The certain allowed parts of blood are from stored blood. Where in the Bible does it say you can have this part of blood but not that part?
Watchtower 6/1/1990 page 31 Questions From Readers
Others have felt that a serum (antitoxin) such as immune globulin containing only a tiny fraction of a donor’s blood plasma and used to bolster their defense against disease is not the same as a life-sustaining blood transfusion. So their consciences may not forbid them to take immune globulin or similar fractions.* They may conclude that for them the decision will rest primarily on whether they are willing to accept any health risks involved in an injection made from others’ blood.
That some protein fractions from the plasma do move naturally into the blood system of another individual (the fetus) may be another consideration when a Christian is deciding whether he will accept immune globulin albumin or similar injections of plasma fractions. One person may feel that he in good conscience can; another may conclude that he cannot. Each must resolve the matter personally before God.
*One example is Rh immune globulin which doctors may recommend when there is Rh incompatibility between a woman and her fetus. Another is Factor VIII which is given to hemophiliacs.