Cass raises his hand; "Cass Sato-Rhodes, sir, Gryffindor 2nd year and Half-blood. As several have mentioned the blood would come from a blood bank where it is checked and tested to be sure it is clean, or that nothing is wrong with it, or from a direct transfusion from a healthy donor of the same blood type to the one that needs the blood." He takes a breath... "Also only a given amount is normally taken so as to not harm the donor in any case. As to the plasma donations, I believe that is the base part of blood. The part that moves it around the body, and the 'red' part is the part that helps keeps us alive."

Cass is rather glad that his father and mother had him take that emergency responder course over the summer, but then his mother's family while wizards always emphasized learning how to fit in to a muggle city properly...