O-neg blood: Blood is classified into groups according to antigens present in the red blood cells. The two main antigens are designated A and B, which gives rise to 4 blood groups: having A only (A), having B only (B), having both (AB), and having neither (O). Each of these groups may or may not contain the rhesus factor. Correct typing of blood groups is vital in transfusion, since incompatible blood will result in blood clotting and possibly death. O-negative blood is universal donor blood; in trauma situations when there is no time for blood typing, O-negative blood can be given without negative consequences.