Blood transfusion. Blood bag being used in a blood transfusion. Blood is made up of many different components and can be classified into different blood groups. This blood is labelled blood group O. This is the universal donor blood group, which means that anyone can receive blood of this group. This grouping of blood is needed, as the mixing of some different blood groups can cause fatal clots to form. This blood also has the Rhesus D antigen, and is labelled Rh-D positive. Normally, only the red blood cells and fluid are needed. The white blood cells (leucocytes) are generally removed, as here, which produces leucocyte-depleted blood. Photographed in the UK.