Blood groups and the Rh factor

Why a transfusion can fail

A person who has lost much blood in an accident may need blood from someone else. This is a blood transfusion. In the early days, it often killed the patient, and no one knew why. Then Karl Landsteiner found that the bloods of people differ. In 1900 and 1901 he described the groups A, B and O. The fourth group, AB, was found a year later by two other workers.

The ABO groups

The surface of a red cell can carry marker molecules called antigens. Two antigens matter here: antigen A and antigen B. The plasma can hold antibodies, which are proteins that attack an antigen. There are two: anti-A and anti-B.

The rule is simple. A person never has an antibody against the antigen that his own cells carry. Otherwise the blood would attack itself.

• Group A: antigen A on the red cell, anti-B in the plasma. It can receive from A and O.
• Group B: antigen B on the red cell, anti-A in the plasma. It can receive from B and O.
• Group AB: antigens A and B on the red cell, no antibody in the plasma. It can receive from AB, A, B and O.
• Group O: no antigen on the red cell, anti-A and anti-B in the plasma. It can receive from O only.

If an antigen meets its antibody, the red cells clump together. This is called agglutination. The clumps block small vessels, and the patient is in danger.

How to read the table

Look at two things only: the antigens of the donor and the antibodies of the recipient. The transfusion is safe if none of the donor antigens meets its antibody.

• Group O has no antigens, so its blood meets no antibody. Group O is the universal donor.
• Group AB has no antibodies, so it attacks no donor cells. Group AB is the universal recipient.
• Group O can receive only from O, because it has both antibodies.

The groups are inherited. You will meet the genetics of the three alleles in Class 12.

The Rh factor

Another antigen was found in the red cells of the Rhesus monkey, and the same antigen was found in many people. It is called the Rh factor. NCERT gives about 80 per cent of people as having it. They are Rh positive (Rh+). The others do not have it, and they are Rh negative (Rh-). The share differs between peoples: about 85 per cent in people of European descent, and over 99 per cent in East Asia.

An Rh negative person does not have anti-Rh antibodies from birth. The body makes them only after Rh positive cells have entered the blood. So a first transfusion of Rh positive blood into an Rh negative person is not harmful. A second one may be.

Rh incompatibility in pregnancy

This matters most when an Rh negative mother carries an Rh positive foetus.

1. In the first pregnancy, the blood of the mother and of the foetus does not mix. At the time of delivery, a small amount of foetal blood may enter the mother.
2. The mother starts to make anti-Rh antibodies.
3. In a later pregnancy with an Rh positive foetus, these antibodies pass from the mother into the blood of the foetus. They destroy the red cells of the foetus.

The foetus or the baby can have severe anaemia and jaundice, and the baby may even die. This condition is called erythroblastosis foetalis.

It can be prevented. The mother is given anti-Rh antibodies immediately after the delivery of the first child. They remove the Rh positive cells of the baby before her own body can make antibodies. In hospitals today the dose is given within 72 hours of the birth of an Rh positive baby, and often at about 28 weeks of the pregnancy as well.

There is no risk if the mother is Rh positive, or if the foetus is Rh negative.

You should find that group O gives to everyone, group AB receives from everyone, and group B cannot give to group A.

Try it in Python

The program uses sets of antigens and antibodies to decide whether a donor can give to a recipient.

Practise with new numbers

Common mistakes

• Saying that group O has no antibodies. Group O has no antigens. It has both antibodies. Group AB has no antibodies.
• Looking at the antibodies of the donor. Check the antigens of the donor against the antibodies of the recipient.
• Saying that the first Rh positive baby of an Rh negative mother is in danger. The first baby is usually safe. The later ones are at risk.
• Giving the anti-Rh antibodies to the baby. They are given to the mother, immediately after the first delivery.
• Saying that an Rh positive mother with an Rh negative baby has the problem. She does not.

Remember these

• ABO: antigens on the red cells, antibodies in the plasma. A has antigen A and anti-B. B has antigen B and anti-A. AB has both antigens and no antibodies. O has no antigen and both antibodies.
• Universal donor: O. Universal recipient: AB.
• NCERT gives about 80 per cent of people as Rh positive. The share differs between peoples.
• Rh negative mother, Rh positive foetus: anti-Rh antibodies form at the first delivery and harm a later foetus. This is erythroblastosis foetalis.
• Prevention: anti-Rh antibodies to the mother immediately after the first delivery.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What are the antigens and the antibodies of a person of group B?

2. Why is group O called the universal donor?

3. Why is group AB called the universal recipient?

4. An Rh negative woman has an Rh positive first baby. Why is the baby usually safe?

5. How is erythroblastosis foetalis prevented?

References

• Landsteiner K. (1901). Über Agglutinationserscheinungen normalen menschlichen Blutes. Wiener klinische Wochenschrift, 14, 1132 to 1134.
• Daniels G. (2013). Human Blood Groups, 3rd edition, Wiley-Blackwell, chapter on the Rh blood group system.
• American College of Obstetricians and Gynecologists (2017). Practice Bulletin No. 181, Prevention of Rh D alloimmunization in pregnancy. Obstetrics and Gynecology, 130, e57 to e70.
• NCERT (2023). Biology, Textbook for Class XI, chapter 15.