This is a particularly ambiguous question as most people are only aware of the ABO and Rh blood groups. These blood groups type us into several categories. Firstly, we are either A, B, AB or O, and additionally either Rh positive or Rh negative. The most common of these groups is O Rh positive, which accounts for approximately 37% of the population.
People with the blood group type O are the most abundant; their red blood cells do not contain any antigens on the cell surface but they have anti-A and anti-B antibodies in the plasma. Type A members contain A antigens on their red cell surface and anti-B in their plasma. Members of group B have B antigens coating the red cell surface and anti-A in the plasma. Finally, AB recipients have both A and B antigens on the red cells and no antibodies in the plasma. Due to our knowledge of these facts it is vital to ensure the correct blood type is given to a person in the instance of a transfusion. These facts mean that people in the blood group AB can receive packed red cell transfusions from any of the other ABO groups, where as group O can donate to any other group.
Those people who are Rh positive have D antigens coating the red cell surface, and those who are Rh negative do not. Therefore, when administering a blood transfusion it is vital to type the recipient's blood before going ahead with the procedure. Those people with Rh negative blood cannot receive blood from an Rh positive donor as it will be incompatible, resulting in a transfusion reaction. However, Rh positive recipients can receive Rh negative blood in an emergency as the red blood cells in the donation will not possess foreign antigens.
From our knowledge of both the ABO and Rh blood groups, we can determine a universal donor, that of type O Rh negative. This blood type can be given via blood transfusion to anyone in an emergency situation. If there is time to determine a person's blood group type, however, this is not recommended as certain reaction problems could still occur.
There are other blood group systems including Duffy, Kell, Lewis, Fy, Jk and P. These are less clinically significant than ABO and Rh due to their decreased prevalence in the population. When typing an unknown person's blood, antibody screens are always performed to detect the presence of any other antibodies in their plasma. Examples include anti-K, anti-Lua or anti-Jka. It is important to discover these antibodies as in the case of transfusion, major complications could occur without this knowledge.