Question: "What is a medical prohibition for Witnesses?"  Context: "Though Jehovah's Witnesses do not accept blood transfusions of whole blood, they may accept some blood plasma fractions at their own discretion. The Watch Tower Society provides pre-formatted durable power of attorney documents prohibiting major blood components, in which members can specify which allowable fractions and treatments they will personally accept. Jehovah's Witnesses have established Hospital Liaison Committees as a cooperative arrangement between individual Jehovah's Witnesses and medical professionals and hospitals."  Answer:
The answer to this question is:
blood transfusions of whole blood