Q.What is the Rh factor? Briefly explain why Rh incompatibility is usually a concern only from the second Rh-positive pregnancy of an Rh-negative mother onward, not the first.

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 foeta …

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. …

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 t …