It has been recently been shown that convalescent plasma from previously infected patients can be used to help others gain immunity against the disease. But why do we need to use plasma for this purpose? Wouldn't a simple blood transfusion work too given that it contains plasma and antibodies?
The Red Cells can be used for other purposes like blood transfusions. It's not necessary for whole blood transfusions as far as we know.
Plasma is the part of blood that remains after the red cells, white cells and platelets are removed. Blood type AB people are considered universal plasma donors because they don't carry Anti-A or Anti-B in their plasma. Whereas Group O negative people are considered universal red cell donors because their red cells lack the A and B antigens.
However, there is some data to suggest the virus can affect haemoglobin synthesis but whether that is meaningful, no one knows.