Blood can be called as a complex living tissues containing different cells and proteins. Blood is a transporter, defender and regulator. Blood carry out the important function of the body. On the surface of the red blood cell a type of antigen called aggulutinogens are attached. Aggulutinogens attached are of two different types A and B. Each of these types has different properties. The ABO is the blood type classification system. This ABO classification system is used to diagnose the presence and absence of these molecules in order to divide blood into four types. Each blood type is either positive or negative. This positive and negative is specified by the presence or absence of RH proteins. If a blood type is positive it means it has RH proteins. If the blood type is negative it means it has no RH protein. Example is if a person has B+ blood type, it means its red blood cell surface has both B and RH proteins.

The A and B antigens on the surface of the red blood cell is produced by the two types of different enzymes. The two enzymes which produce two antigens A and B on the surface of the red blood cells are encoded by different version of the same gene A and B. These copies of the genes are inherited from parents. Following are the combination of alleles producing different blood types A+A=A, A+O=A, A+B=AB, B+B=B, B+O=B and O+O=O. Blood plasma are packed with proteins. These proteins are called antibodies. The body produces variety of antibodies. The function of the antibodies is to recognize and attack the foreign molecules that enter the body from outside. While blood transmission it is important that the blood group of recipient and the donor match. If the donor blood cells surface is different from the recipient, then the antibodies in the recipient’s blood recognize the donor blood as a foreign. As result immune response take place which result in blood clotting. Person having O blood group are the universal donors because they don’t have any molecule on the surface of their red blood cells. If the donors red blood cells have the same surface molecules as of the recipients then they are not recognized as foreign and will not result in an immune response.