Blood Groups

Blood Groups

Blood group is inheritable. ABO blood system in humans is triallelic system that generates six genotypes. The loss of blood due to accident or disease creates blood deficiency. These persons need blood transfusion from another person (donor). The process needs to be carefully monitored because blood groups need to be properly matched.

Karl Landsteiner (1868-1943) discovered that during blood transfusion, the blood of donor should match the blood of the recipient. In case the blood of both does not match, the blood of donor agglutinates on mixing with the recipient’s blood which may lead to the death of the recipient. The agglutination infact takes place in the red blood corpuscles (RBC’s) of the donor.

Blood Proteins

According to Karl Landsteiner (1900), a Noble Prize winner, blood contains two types of proteinous substances responsible for agglutination.

(A). Agglutinogen or antigen: It is a protein found on the cell membrane of RBC’S.

(B). Agglutinin or antibody: This is the other protienous substance, found in the plasma of the blood.

Whenever the blood of a person receives foreign proteins (antigen) his blood plasma starts forming antibodies in order to neutralize the foreign antigen.


Two types of antigen occur on the surface of the red blood corpuscles of man, which is termed as antigen A and B. These antigens react with the two types of antibodies found in the blood plasma termed as antibody-anti A or ‘a’ and anti-B or ‘b’. Agglutination takes place only when antigen A and antibody ‘b’ occur together in the blood. Antibody ‘a’ reacts with antigen A and makes it highly sticky.

Similarly antigen B in the presence of antibody ‘b’ becomes highly sticky.  RBC’s containing these antigens clump causing blockage of the capillaries. Some have only one antigen , others have both and still others have none of them. Agglutination in blood is, therefore, antigen-antibody reaction.

Types of Blood Groups

Landsteiner divided human population into four groups based on antigens occuring in their red blood corpuscles. Each group represented a blood group. Thus, there are four blood groups viz. A, B, AB and O. He observed that there was a reciprocal relationship between antigen and antibody. A person has antibodies for those antigens which he does not possess. For example, a person of blood group B does not possess antigen A but his blood plasma has antibody ‘a’.

Similarly persons with blood group AB possess both the antigens A and B but their blood plasma does not possess any of the antibodies. In the same way person having blood group A does not possess antigen B but antibody ‘b’ occurs in his blood plasma. Human being with blood group O possess none of the antigens and that is why their blood possesses both the antibodies ‘a’ and ‘b’.

Inheritance of blood groups

Blood groups in human are inheritable traits and are inherited from parents to offspring following Mendel’s laws. Blood group inheritance depends on genes received from parents. There are three genes (instead of two) which control blood groups. These are termed as multiple alleles. The three (genes or alleles) are located on the same locus on homologous chromosomes. A person can have only two of the three genes at one time which may be either similar or dissimilar.

These genes control the production of blood group (antigen) in the offspring. A gene producing antigen A is denoted by  IA, gene for antigen B by IB and the gene for the absence of both antigens by IO. Letter I (Isohaemagg lutinogen) is a basic symbol for the gene. It is sometimes represented by L (for Landsteiner). Based on this six types of genotypes are possible for four blood groups and human population.

Gene  IA and IB are both dominant over IO, but not over each other. The blood groups are inherited in the simple Mendelian fashion. Thus offspring with all four kinds of blood groups are possible if the parents are heterozygous for blood groups A and B. In the same way a cross between, parents heterozygous for blood group A can have their offspring of blood group A and O both. If the parents are heterozygous for group B, their genotype will be IB IO and the offspring can have blood group B and O.

When the parents have blood group O, their offspring will be of blood group O only. In case of parents with blood groups A, B, AB and O, the offspring will also be a blood group A, B, AB and O respectively. If we know the blood groups of a couple the blood groups of the children can easily be predicted.

Rh Factor and its Inheritance

Landsteiner and Wiener discovered a different type of protein in the blood of Rhesus monkey. They termed it as Rh antigen or Rh factor . Injection of  blood of these monkeys into the blood of guinea pigs, results in the formation of antibodies  against the Rh antigens in their blood.  Introduction of Rh serum containing antibodies into the blood of human results in the agglutination of human blood with Rh serum.

Formation of Rh antigen is controlled by dominant gene (R) and its absence by recessive gene (r). People having this antigen with genotype ( RR or Rr) are termed as Rh positive and those whose blood is devoid of it with the genotype (rr) are Rh negative. About 85% human beings in Europe and 97% in India are Rh positive.

Blood Groups

Fig: Rh Factor

Importance of Rh factor

On transfusion of blood of a Rh positive person to Rh negative person, the recipient develops Rh antibodies in its blood plasma. Transfusion of Rh positive blood for the second time causes agglutination and leads to the death of Rh negative person.

Erythroblastosis Foetalis

This disease is concerned with the birth of a child related with Rh factor. It causes the death of a foetus inside the woman or just after birth. It was studied by Levine together with Landsteiner and Wiener. The father of Rh affected foetus is Rh positive and the mother is Rh negative. The child inherits the Rh positive trait from the father. A few Rh positive red blood corpuscles of foetus in the womb enter the blood of the mother where they develop Rh antibodies. As mother’s blood is Rh negative i.e., devoid of Rh antigen, it causes no harm to her. These Rh antibodies along with the mother’s blood on reaching the foetal circulation cause clumping of foetal RBCs or agglutination reaction.

The first child is somehow born normal because by that time the number of antibodies in the mother’s blood remain lesser but they increase with successive pregnancies. Thus the foetus following the first child dies inside the womb or just after its birth. This condition is termed as erythroblastosis foetalis.

A marriage between Rh positive boy and Rh negative girl is biologically incompatible. Rh positive person can donate blood to Rh positive person. Person with Rh negative can donate to Rh negative person. Rh negative person can donate to Rh positive person. But Rh positive person cannot donate blood to Rh negative person. Hence universal donor is O negative blood group.

However, there is no danger if both parents are Rh negative or mother is Rh positive and father is Rh negative. Rh factor serum given to Rh negative mother after each childbirth saves the next child. This serum contains Rh antibodies which destroy the antigens of the foetus before they can initiate formation of Rh antibodies in the mother.