The meaning of blood group types is classification of the blood of different individuals on the basis of the antigenic nature of the red cells. . Before going into the detail of blood group type it is necessary to explain the terms antigen and antibody.
What is antigen?
An antigen is a substance, usually a protein or muco-polysaccharide, which is foreign to the blood of an animal and stimulates formation of a specific antibody.
What is antibody?
An antibody is a protein produced in the body in response to stimulation by an antigen and reacts specifically with that very antigen to inactivate and destroy it. In general, antibodies are produced only when the blood is exposed to an antigen. But unexpectedly, some blood group antibodies are exception to this rule as they occur naturally in blood without a previous exposure of the blood to their antigens.
What is agglutination?
When incompatible blood samples are mixed together, there occurs clumping of red cells. This is referred to as agglutination which is a manifestation of antigen-antibody reaction and is followed by hemolysis i.e., rupture of red cells and causes death. The red cells in blood contain certain antigens called agglutinogen on their surface, on the other hand the plasma or serum contains antibodies or agglutinins. Depending upon the occurrence of agglutinogens, persons have been classified into blood group types. There are two main ways of blood grouping, which are the ABO system and the Rh factor.
ABO system of blood group types
In the Red Blood Cell membrane, there are two primary agglutinogens named A and B. Their equivalent antibodies present in plasma are termed α (or anti A) and β (or anti B). The red cells of a particular person may contain only A or only B or both A and B or none of them. Similarly the plasma may contain only α or only β or none of these. The antibodies of the ABO blood group types system happen naturally i.e., they are found to be present in human plasma impulsively, without being induced by the antigen.
Human beings are classified into the following four blood group types according to the agglutinogen present in their red cells :
(i) Group A-containing only A ;
(ii) Group B-possessing only B ;
(iii) Group AB-having both A and B
(iv) Group O-having neither A nor B.
Compatibility incompatibility during transfusion on the basis of blood group types
If the blood of the donor and the recipient react with each other to cause agglutination, the groups are called incompatible or mismatching and on the other hand, there is no such reaction, the groups are called compatible or matching. Now look at the following table which shows the type of agglutinogens and agglutinin present in different groups of blood.
In case of an incompatible transfusion, there are two possible ways of agglutination :
i) Donor's plasma may agglutinate recipient's red cells
ii) Recipient's plasma may agglutinate donor's red cells.
The first (No. i) possibility is unimportant because the antibody present in the donor's plasma is well diluted by the recipient's plasma. Therefore, the reaction between donor's plasma and recipient's red cells is called minor reaction. On the other hand, the antibody present in the recipient's plasma is not considerably diluted by the donor's plasma (which is relatively much smaller in volume) and therefore it can agglutinate the donor's red cells. So, the reaction between the recipient's plasma and donor's red cells is called major reaction.
Compatibility of blood group types
Considering the above facts, the possibility of agglutination reaction between donor's red cells and recipient's plasma of different blood group types we can conclude the following facts: -
(1) Group O blood can be transfused to any group as it does not contain any agglutinogens at all, hence it is called universal donor.
(2) Group AB can receive the blood of any group because it contains no agglutinin; hence this group is called universal recipient.
(3) Same groups are always compatible and hence can be transfused safely.
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