About Human Blood Group

Human blood is grouped based on the presense or absense of inherited antigenic (a molecule that sometimes stimulates an immune response) substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins or glycolipids, depending on the blood group system.

Most commonly known blood group system is ABO blood group system and Rhesus blood group system. ABO blood group system has A-antigen and B-antigen. The presense or absense of these two antigens will determine the blood group as A, B, AB or O (see figure below). Rhesus blood group system uses RhD antigen. The presense or absense of this antigen will determine Rh+ or Rh- blood group. Combining these two blood group systems, one can have 8 possible kind of blood groups named as A+, A-, B+, B-, AB+, AB-, O+ and O-.

Apart from antigen, blood also has antibodies, which will protect us from external bodies such as bacteria and viruses. It is a part of human immune system. As shown above, corrosponding antibodies are also present in perticular blood group. Blood group A will have antibody anti-B. Blood group B will have antibody anti-A. Blood group O will have antibodies anti-A and anti-B. Blood group AB will not have any of these antibodies (anti-A and anti-B). So, if during blood transfusion, if blood groups are not matching, these antibodies will bind to other blood's antigen and will damage the red blood cells. This can can lead to a more vigorous immune response with massive RBC destruction, low blood pressure, and even death. This explains why blood group AB is universal accepter and blood group O is universal donor. Since blood group AB is having no antibodies, it will not react to any kind of blood and thus can accept any kind of blood group. Similarly, blood group O is not having any antigen present, so any antibodies can not react with this type of blood and thus it can be transfused to any kind of blood group. But since, blood group O has both kind of antibodies, it can not accept A, B or AB blood group.

Blood group AB: Individuals have both A and B antigens on the surface of their RBCs, and their blood serum does not contain any antibodies against either A or B antigen. Therefore, an individual with type AB blood can receive blood from any group (with AB being preferable), but can donate blood only to another group AB individual.

Blood group A: Individuals have the A antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the B antigen. Therefore, a group A individual can receive blood only from individuals of groups A or O (with A being preferable), and can donate blood to individuals of groups A or AB.

Blood group B: Individuals have the B antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the A antigen. Therefore, a group B individual can receive blood only from individuals of groups B or O (with B being preferable), and can donate blood to individuals of groups B or AB.

Blood group O: Individuals do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM anti-A antibodies and anti-B antibodies against the A and B blood group antigens. Therefore, a group O individual can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group (ie A, B, O or AB). If anyone needs a blood transfusion in a dire emergency, and if the time taken to process the recipient's blood would cause a detrimental delay, O Negative blood can be issued.

Recepient	Donner
	O-	O+	A-	A+	B-	B+-	AB-	AB+
O-	Yes	No	No	No	No	No	No	No
O+	Yes	Yes	No	No	No	No	No	No
A-	Yes	No	Yes	No	No	No	No	No
A+	Yes	Yes	Yes	Yes	No	No	No	No
B-	Yes	No	No	No	Yes	No	No	No
B+	Yes	Yes	No	No	Yes	Yes	No	No
AB-	Yes	No	Yes	No	Yes	No	Yes	No
AB+	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes