Watchtower’s official view:
“You also ask why one can be disfellowshipped for taking a blood transfusion but not for taking blood fractions. While both may affect the life of an individual, the expression "life-sustaining" in connection with blood transfusions is synonymous with the idea of taking in food for nourishment . In this regard both whole blood and major components of it carry nutrients, oxygen, and other nourishment to the body. It is this aspect of taking in blood, that is, to provide nourishment that links blood transfusions with the Biblical prohibition . Note that "Questions From Readers" of the July 1, 1975, issue of The Watchtower stated: " The Bible specifically forbids the taking of blood to nourish the body .-Gen 9:4; Lev. 17:1-14; Acts 15:28, 29." The motive or reason for taking a serum is significantly different. It is not to feed the body, as would be the case if there was an eating of whole blood (or a major component thereof) by mouth or by having it transfused intravenously. Rather, the antibodies that have been separated out are administered for the purpose of immunizing the body against a certain disease. While blood fractions in certain situations can be lifesaving, they do not operate to feed and nourish the body and in this way sustain life but, rather, utilize other mechanisms.” [From March 23, 1998 letter from Watchtower Bible & Tract Society to R. Jensen.]
Another view:
“A major question, then, is whether it can be demonstrated that the transfusing of blood is an “eating” of blood as the Watch Tower organization claims. There is, in reality, no sound basis for such claim. There are, of course, medical methods of “intravenous feeding” whereby specially prepared liquids containing nutrients, such as glucose, are introduced into the veins and provide nourishment. However, as medical authorities know, and as the Watch Tower Society has at times acknowledged, a blood transfusion is NOT intravenous feeding; it is actually a TRANSPLANTATION (of a fluid TISSUE), not an infusion of a nutrient. In a kidney transplantation, the kidney is NOT eaten as some food by the new body it enters. It remains a kidney with the same form and function. The same is true of blood. It is not eaten as food when “transplanted” into another body. It remains the same fluid tissue, with the same form and function. The body cells cannot possibly utilize such transplanted blood as food. To do this the blood would first have to PASS THROUGH THE DIGESTIVE SYSTEM, be broken up and prepared so that the body cells could absorb it—thus it would have to be actually and literally EATEN to allow it to serve as FOOD.” [From IN SEARCH OF CHRISTIAN FREEDOM, pages 297 & 298, by Raymond Franz.]