Q.Explain the basis of the ABO blood grouping system, naming the antigens and antibodies associated with each of the four blood groups.

Erythroblastosis foetalis is caused by maternal anti-Rh antibodies destroying the foetus's Rh-positive red blood cells, an example of Rh incompatibility between mother and foetus.

The Rh factor is an antigen present on the surface of red blood cells in Rh-positive individuals and absent in Rh-negative individuals. If an Rh-negative mother carries an Rh-positive foetus (having inherited the Rh antigen from an Rh-positive father), some foetal red blood cells can leak into the mother's circulation, usually around childbirth or a placental injury, and the mother's immune system responds by producing anti-Rh antibodies. In a subsequent Rh-positive pregnancy, these maternal antibodies can cross the placenta and attack and destroy the foetus's red blood cells (RBCs), a condition called erythroblastosis foetalis (also called haemolytic disease of the newborn). This destruction of foetal RBCs can cause severe anaemia, jaundice, and in serious cases can be fatal to the foetus/newborn — which is why Rh-negative mothers are given an anti-D (Rh immunoglobulin) injection to prevent this immune sensitisation.

A person whose RBCs carry both antigen A and antigen B on their surface belongs to blood group AB.

In the ABO blood grouping system, blood group is determined by the presence or absence of two antigens (agglutinogens), A and B, on the surface of red blood cells:

• Blood group A: only antigen A present.
• Blood group B: only antigen B present.
• Blood group AB: both antigen A and antigen B present.
• Blood group O: neither antigen A nor antigen B present.

Since the question specifies a person having BOTH antigen A and antigen B on their RBCs, that person belongs to blood group AB. (AB individuals are also known as 'universal recipients' since their plasma lacks both anti-A and anti-B antibodies.)

Surface antigens on red blood cells, which determine blood group and can trigger clumping with a mismatched antibody, are called agglutinogens.

Human blood is grouped mainly on the basis of specific glycoprotein/glycolipid molecules present on the plasma membrane of red blood cells. These surface molecules act as antigens because the immune system of a person lacking them will recognise them as foreign; they are specifically termed agglutinogens (e.g., agglutinogen A and agglutinogen B in the ABO system). Circulating in the blood plasma are corresponding antibodies called agglutinins (e.g., anti-A, anti-B) which are NOT present against a person's own agglutinogens (to avoid self-clumping) but ARE present against the agglutinogens that person lacks. When agglutinogen-bearing RBCs of one blood type are mixed with plasma containing the matching agglutinin (as in a mismatched transfusion), the antibody binds the antigen and causes the RBCs to clump together — agglutination. Thrombocytes are platelets involved in clotting, and microglia are immune cells of the nervous system, not RBC surface molecules — both are unrelated to this question.