ABO and Rhesus Systems

The ABO System

Erythrocytes may have one of 3 different antigens on their surface

These antigens are called A, B and AB and blood with erythrocytes carrying these antigens are categorised as blood group A, blood group B and blood group AB. Blood which has neither A or B antigens belongs to blood group O.

Antigens

Antibodies

An antibody is a protein produced by the body's immune system in response to antigens

Antigens include microbes and toxins.Blood group A has A antigens and anti B

antibodiesBlood group B has B antigens and anti A

antibodies

Antibodies

Blood group AB has neither anti A or anti B antibodies.

Blood group O has no antigens but has anti A and anti B antibodies.

This is summarised in the table below.

Blood groups and genotypes

Agglutination reactions

Agglutination occurs when incompatible blood groups are mixed together and this results in the clumping ( not clotting) of blood.

For instance, if blood group A (which has A antigens on its erythrocytes and B antibodies in its blood plasma) is mixed with blood group B (which has B antigens on its erythrocytes and A antibodies in its plasma), the A antibodies would cause agglutination with the A antigens on the red blood cells

Question

What would happen if blood group AB was mixed with blood group B?

What are the antigens in blood group AB?What are the antibodies in blood group

AB?What is the antigen in blood group B?

Answer

Antigens in blood group AB are antigens A and B.

Antibodies in blood group AB are antibodies A and B

Blood group B has B antigens and A antibodies

So the A antibodies in blood group B will react with the A antigen in blood group AB causing clumping or agglutination.

Agglutination Reactions in the ABO System

Blood type Anti B Serum Anti A Serum

A No agglutination Agglutination

B Agglutination No agglutination

AB Agglutination Agglutination

O No agglutination No agglutination

The Rhesus System

Red blood cells can also have the Rhesus antigen or not. If the red blood cells have the Rhesus antigen then the blood is Rhesus positive and the blood is Rhesus negative if it lack Rhesus antigens

Haemolytic disease of the new born (HDN)

If an Rhesus negative mother is carrying her first baby and it is Rhesus positive, there are no problems because maternal and foetal blood do not mix.

During the process of birth however, some foetal blood may leak from the placenta into the maternal blood causing the Rhesus negative mother to produce antibodies against the Rhesus antigen.

Haemolytic disease of the new born (HDN)

If this Rhesus negative mother then gets pregnant and her second baby is also Rhesus positive, the Rhesus antibodies in the mother’s blood will attack the baby causing haemolysis of the baby’s red blood cells. This may lead to the death of the baby unless it is given frequent blood transfusions

Haemolytic disease of the new born (HDN)

To prevent this, the mother needs to be given an injection of anti Rhesus antibodies after every delivery, miscarriage or abortion.

These antibodies bind to foetal Rhesus antigens and inactivate them. Consequently, the mother does not produce any anti rhesus antibodies herself which would harm subsequent Rhesus positive foetuses.

Haemolytic disease of the new born (HDN)