Blood Group and HDN

Adapted from Haematology, an illustrated color text

The blood groups
Antigen exits on RBC membrane
Numerous blood group systems encoded by genes on different chromosomes
highly variable in their polymorphonism and clinical significance
most important is ABO system, the genes encodingis located on chromosome 9 and inherited in autosomal dominant fashion
Each antigen is a sugar residue made by a specific glucosyl transferase

This is an example of antigen H. which means no extra added residue, and this is antigen for blood group type O.

This is example of antigen A, which has added N-acetylgalactosamine to the original template antigen H. This is antigen for the blood group type A.

This is antigen B on surface of rbc blood group type B. It has another added galactos on the template antigen H.

Person who has blood group type AB is having both antigen A and antigen B on their RBC surface.

The ABO SYSTEM is crucial in blood transfusion. As the antibody against ABO antigen is naturally occuring. For example, if you are blood group type A, this means that you have antigen A on your red cell, but you do not have antigen B on your red cell, so your body will naturally have antibody against B, which we called it as anti-B.

These naturally occurring antibodies(anti-A and anti-B) are of IgM type and if the blood transfusion is not compatible, the patient serum which contains this IgM will attacks donor's red cell and will induce fatal hemolytic transfusion reaction.

So test must be done before blood transfusion. We will discuss this later on.

Another type of antigens are minor antigens. ABO is considered as major but our RBC surface might also have other antigens namely Rhesus, Duffy, Lewis, Kell etc. The antibody against these antigens are not naturally occurring, they are called 'immune antibodies' as they will only be produced when your body's immune system is sensitized by these antigens. The strongest antigen D(Rhesus), for example, can be induced through previous blood transfusion or pregnancy. In pregnancy, if the mother is Rhesus -ve but baby is Rhesus +ve(inherited from father), during first pregnancy, some of the fetal blood might leak into the mother during delivery and causes sensitization of the mother's immune system. Once mother's blood contains this antibody, the second baby which is also Rhesus +ve, the fetal antigen will be attacked by mother's developed antibody and thus develop hemolytic newborn disease(HDN). The HDN here must not be confused with Hemorrhagic Newborn Disease, which is increased risk of bleeding in newborn due to vit K deficiency which impaired the production of Factor 2, 7, 9, 10, C and S by the liver.

The reasons that being different in Rhesus grouping between mother and child has more consequences than ABO grouping is, 1) the rhesus antibody is lgG in nature and can cross placenta to reach the fetus whereas ABO antibody(IgM) can't 2) Fetus doesn't develop fully yet ABO antigen, so less likely to sensitize mother and cause an immune attack.

Notes to come: GSH, GXM, Blood Grouping, Coombs test