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There are two different types of chemicals on the surface of the red blood cells. These chemicals are  called antigens and are named as A and B. Two corresponding antibodies are also present in the plasma. Based upon the presence of A or B antigens, blood groups are classified  as A, B, AB and O. 
4 types of blood group can be formed by these alleles and they are: 
1. Blood group A in which the allelic combination is either AA or Ai.
 2. Blood group B in which the allelic combination is either BB or Bi.  
3. Blood group AB in which the allelic combination AB. 
4. Blood group O in which the allelic combination is ii.   

Blood type must be determined before blood is transfused into a patient to prevent a transfusion reaction. When someone has type A blood (with type A antigens), their plasma has type B antibodies. Therefore, when the antigens on the red blood cells of the donor blood react with the antibodies in the recipient's plasma, a reaction occurs. For example, if a unit of blood type A (which contains A antigens) is transfused into someone with type B (they have anti-type A antibodies in their blood), a transfusion reaction will occur. When a transfusion reaction does occur, an antibody attaches to antigens on several red blood cells and result in causes the red blood cells to clump together and plug up blood vessels. This can cause jaundice and may prove fatal. AB+ blood  group is universal acceptor. O blood group is a universal donor