Our red blood cells (and some tissues) have got chemical substances called antigens on their surface and the ability to form these antigens is governed by genes inherited from parents. These antigens may be proteins, carbohydrates or other complex chemicals. The presence of these antigens (and their antibodies) has given rise to blood group systems and they play a role in blood transfusion and tissue typing. Currently about 30 different blood group systems are known in humans but the ones of clinical significance are the ABO system, Rh system, Kell, MNS, Lewis etc. The importance of blood group systems lies in transfusion and transplant medicine as we can receive blood (or organ) from only an individual whose blood group matches ours. In case of mismatch, the bodys immune system recognises the foreign antigen and fights it leading to disease states. Thus, blood group matching is done so that compatible blood (or tissue) is selected. There are four possible blood groups in the ABO system: AB, A, B and O. Blood group A individuals possess A antigen on their red cells and have naturally occurring antibodies (anti-B) in their serum that react with group B and AB people. Blood group B individuals possess B antigen on their red cells and have naturally occurring antibodies (anti-A) in the serum that react with group A and AB people. Individuals with group AB possess both A and B antigens on their red cells and produce no naturally occurring antibodies. Finally, individuals with group O have neither A or B antigens on their red cells and have naturally occurring antibodies (anti- A and anti - B) in their serum that react with red cells from group A, B and AB people. The Rh (rhesus) antigen was first discovered in the rhesus monkey and thus is so named. Individuals carrying this antigen are called Rh-positive (Rh+) while those lacking it are Rh-negative blood group type (Rh-).