way blood typing works is this: All blood cells are the same inside, but the outsides are covered with different kinds of antigens—that is, proteins that project outward from the cell surface—and that is what accounts for blood types. There are some four hundred kinds of antigens altogether, but only a few have an important effect on transfusion, which is why we have all heard of types A, B, AB, and O, but not, say, Kell,
