As biological individuals, there is something about each of us which makes us different from everyone else. Apart from differences in appearance, personality, behavior, certain of the larger molecules which make up our bodies are also different in chemical structure between one individual and another. In connection to blood groups or typing, certain protein molecules of the plasma called antibodies are present in some bodies or absent in some bodies.

So, also the case with antigens. Antigens are substances which are capable of stimulating the production of antibodies. Therefore antigen and antibodies react with each other. There are two related antigens and antibodies called A and B which are the basis of the blood groups.

Blood from one person cannot always be safely mixed with that of another, because of the transfusion reaction which could occur due to the presence or absence of agglutinogens and agglutinins in blood. All human beings can be divided into four groups A, B, AB and O, depending on the type of agglutinate found in their red corpuscles. Agglutinogens found in red cells are substances which have the power of stimulating the production of agglutinins (antigens).

And agglutinogens found in blood serum are substances which have the power of causing the red cells of persons belonging to another group, to run together in clumps or agglutinate, if they are mixed with this serum.


In blood transfusion, it is necessary first to find which group he belongs to and then to find a donor belonging to the same group. There are two agglutinogens, called A and B. If A is present, the blood group is called Group A, if B is present the blood group is called Group B, if both A and B are present, the blood group is called Group-AB and if neither agglutinate is present the group is called Group O.

Next, the plasma of each blood group contains substances called agglutinins, which causes agglutination (clumping or becoming sticky, of red cells if incompatible blood groups are mixed. Agglutinins are called anti-A and anti-B and plasma contains all agglutinins which will not affect its own red cells. Therefore, the plasma of Group A contains anti-B agglutinin, the plasma of Group B contains anti A agglutinin, the plasma of Group AB contains no agglutinin and the plasma of group O contains both anti A and anti B agglutinin.

Different Blood Groups

Blood Group—Agglutinogen in red cells—Agglutinins in plasma—Transfusion possible


A—A—Anti-B—Group A & O

B—B—Anti-A—Groups B & O

AB— A & B—Neither—Any Group

O— None Anti B—Anti A and—Group O only


When a donor’s red cells are mixed with the recipient’s (who receives the blood) plasma in the laboratory, it can be seen with the help of a microscope whether agglutination occurs. It will be noticed that Group AB has no agglutinins in the plasma, and therefore cannot cause any red cells to agglutinate. This means that a patient belonging to this group will probably be able to receive blood from any other group and hence known as the universal recipient. Group O contains no agglutinogens in the red cells and therefore they cannot be made to agglutinate by the agglutinins in any plasma. Therefore this group blood can probably be given to a patient belonging to any group and is known as the universal donor.

If unmatched or incompatible blood is transfused, the red cells forms clumps and block the blood vessels and obstruct the circulation, proving fatal to the patient.

The frequency of occurrence of the four blood groups is as follows:

Group A – 42% (approx)


Group B – 8% (-do-)

Group AB – 4% (-do-)

Group O – 46% (-do-)

The Rhesus (Rh) Factor:


Apart from the blood groups ABO, another substance was discovered from the blood, called as the ‘Rhesus factor and abbreviated as Rh. This is a component of red blood cells and present in about 25 percent of the population. The individuals who possess this component are said to be Rh positive, and the remaining 15% who do not have this component are said to be Rh negative.

Under certain circumstances antibodies may develop in the blood of an individual which are capable of causing the agglutination and destruction of Rh positive cells. These antibodies are called as the anti-Rh.

Transfusion of RH + VE Blood

If the blood of a person who is Rh + ve is transfused into a person who is Rh- ve, the recipient’s body slowly produces antibody to the agglutinogens. There may be no indication of this incompatibility following the first transfusion, but if a second transfusion is given 10 days or even years later, serious reaction may occur, because the anti-Rh factor may cause damage to the transfused erythrocytes.


Conception of RH + VE Child by RH – VE mother

If two parents are both Rh + VE, their offspring will be Rh positive. If one of the parents is Rh + VE, offspring will probably be Rh + VE, but if the mother is Rh – VE and her child is Rh + Ve, in a certain number of cases, the mother then becomes sensitive to the positive factor in the child’s blood and develops anti-Rh factor. When this substance crosses the placenta into the fetal blood, it causes haemolysis of the fetal erythrocytes. This factor is slow to develop and no serious effects will be noticed in the first child, but in subsequent pregnancies, anti-Rh factor is very likely to develop and destroy the erythrocytes of the Rh + VE baby.

Father Mother Child

Rh + ve Rh + ve Rh + ve = normal

Rh – ve Rh + ve Rh + ve = normal

Rh – ve Rh – ve Rh – ve = normal

Rh + ve Rh – ve Rh + ve = 4% risk of abnormality in the first child