How can a cancer patient's blood disappear after a transfusion?

Your Red Blood cells don't last forever, they breakdown and get ped out through your body as waste. These are the cells that primarily make up blood and carry oxygen to your tissues. Your body should be producing them as fast as they leave. In cancer patients, these mechanisms don't work and all kinds of blood cells are low because of it. Transfusions help give them a boost but don't last because of your bodies natural way of using them.