Blood is a specialized fluid, which is composed of blood cells and a liquid, blood plasma. Blood plays a very important role in the lives of both animals as well as humans.
Blood is composed of two types of blood cells; red blood cells and white blood cells where as blood plasma is a liquid, in which these cells are suspended.
Blood Type System ABO System
For the purpose of human-blood transfusion, several blood type systems have been suggested. ABO system is the most common and relied upon. According to this system, human blood has been categorized into four groups; A, B, AB and O. This classification of blood is based on the presence or absence of inherited antigens on the surface of red blood cells. A person with blood group A has A antigens on the surface of his RBCs and B antibodies in blood plasma. Similarly, a person with blood group B has B antigens on his RBCs and A antibodies in his blood plasma. Whereas a person with blood group AB has both A and B antigens on his RBCs and none of the antibodies in his blood plasma. On the other hand, a person with blood group O has none of the antigens on his RBCs and both A and B antibodies are present in his blood plasma.
Another blood group system that runs parallel to ABO system is the Rh blood group system. In this system, presence or absence of Rh factor in blood is symbolized by positive or negative respectively.
Blood Transfusion Rules
As far as blood transfusion is concerned, there are certain rules which are to be followed strictly. Blood groups with similar antigens can be transfused only. AB+ cannot be transfused into a person whose blood group is O+, because although Rh factor is present in both, yet antigens A and B are absent in O+. Similarly Rh positive can only be transfused into Rh positive e.g. a person with blood group AB- cannot be donated AB+.
In this case, blood group O- is the universal donor because it has none of the antigens on its RBCs and also it lacks Rh factor so it can blend in with any of the blood groups. Hence, this blood type can be transfused into a person with any blood group, irrespective of the absence or presence of any of the antigens, antibodies or Rh factor. Blood group O- is comparatively expensive and hard to find because only one or two persons in a hundred are blessed with this blood type. Also, none of the other blood types can be donated to an O- person where as for all the other blood types; there are at least 2-3 donors’ types. This proves the inestimable worth of the universal donor of all blood groups i.e. O-.