i. Agglutinogen or antigen is a glycoprotein present on the surface of RBCs. It is called the corpuscle factor.

ii. Agglutinin or antibody is a gamma globulin protein present in blood plasma (which is produced by lymphocytes) and is called the plasma factor.

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There are two kinds of antigens A and B, and two kinds of antibodies a and b, in the blood. Antigen A and antibody are antagonistic or incompatible and cause self-clumping. Similarly, antigen B and antibody b are incompatible and cause self-clumping. Antigen A is compatible with antibody b and antigen B is compatible with antibody a.

On the basis of type of antigen present on the surface of RBCs as given below, a system of blood groups known as ABO system in which there are four blood groups is recognized in the human blood.

i. Group A with antigen A and antibody b

ii. Group B with antigen B and antibody a.

iii. Group AB with both A and B antigens but no antibody.

iv. Group O has no antigen but both antibodies a and b.

Blood GroupAntigens on Red Blood CorpusclesAntibodies in PlasmaCan Donate Blood toCan Receive Blood from
AAbA, ABA, O
BBaB, ABB, O
ABABNoneABA, B, AB, O
ONonea, bA, B, AB, OO

Blood Transfusion:

Knowledge of blood groups is essential for safe blood transfusion.

The antigens of the donor’s blood can react with antibodies of the recipient’s blood and cause clumping of RBCs. Thus, antigen A present in the RBCs of blood group A individuals reacts with antibodies of plasma of blood group B individuals and vice versa.

This phenomenon is known as agglutination. Agglutination may cause serious consequences and even prove fatal. However, the RBCs of blood group O individuals lack antigens and are not clumped by antibodies present in the serum of the recipient’s blood.

It means blood group O can be given to persons with blood group O. A. B or AB. Hence persons with blood group O are called universal donors.

However, persons with blood group AB lack antibodies in their plasma, so they can receive blood from A, B, O, or AB blood groups. Such persons are called universal recipients.

Rh-Factor (Rhesus Antigens):

Landsteiner and Wiener discovered in 1940 that the surface of human RBCs contains a protein that is also found in the RBCs of Rhesus monkeys. So, it was termed as Rh antigen or Rh factor. Those persons who have this factor are called Rh-positive and others as Rh-negative.

Both Rh-positive and Rh-negative persons are quite normal. The problem arises when Rh– blood comes in contact with Rh+ blood either due to blood transfusion or during pregnancy.

Incompatibility during Blood Transfusion:

The Rh– blood can be given safely to an Rh+ individual. When Rh+ blood is transfused into Rh” person, the recipient forms antibodies in his/her blood. However, no complications develop after the first transfusion.

In case of a second transfusion of Rh+ blood to an Rh– person, the latter’s anti-Rh factors attack and destroy the donor’s red blood corpuscles. Therefore, it is always advised that the patient’s Rh factor is determined before transfusion.

Incompatibility during Pregnancy:

A serious problem arises if an Rh– mother is carrying an Rh+ foetus. The Rh+ blood of the foetus will stimulate the formation of anti-Rh factors or antibodies in the mother’s blood. During the first pregnancy, enough antibodies are not produced to harm the foetus.

During the second pregnancy, if the foetus is Rh+, more antibodies will be produced in mother’s blood.

Due to the cumulative effect of antibodies produced the second time, in addition to the antibodies already present in mother’s blood, a large number of RBCs of the foetus are destroyed. This causes death of the foetus. This is called erythroblastosis foetalis.