Introduction
There are different types of blood grouping systems; ABO blood group system, the Rh system, H-antigen, Duffy system, Kidd system , MNS antigen system and so many others but the most important and widely used blood group systems are the ABO and Rh system.
The ABO system : there are 4 main blood groups defined by the ABO system:
- Blood group A – has A antigens on the red blood cells(RBCs) with anti-B antibodies in the plasma.
- Blood group B – has B antigens with anti-A antibodies in the plasma.
- Blood group O – has no antigens, but both anti-A and anti-B antibodies in the plasma.
- Blood group AB – has both A and B antigens, but no antibodies.
The Rhesus (Rh) system is the second most important blood group system after ABO system. RBC surface of an individual may or may not have a Rh factor or immunogenic D-antigen. Accordingly, the status is indicated as either Rh-positive (D-antigen present) or Rh-negative (D-antigen absent). In contrast to the ABO system, anti-Rh antibodies are, normally, not present in the blood of individuals with D-negative RBCs, unless the circulatory system of these individuals has been exposed to D-positive RBCs. Hence, when an individual who is Rhesus D negative (Rh D-ve) is exposed to a Rhesus D positive (Rh D+ve) blood the individual develops antibodies against the Rh D+ve blood and this phenomenon is called rhesus isoimmunisation. These immune antibodies are immunoglobulin G (IgG) in nature and hence, can cross the placenta.
Rhesus isoimmunization affects only Rhesus D negative individuals, particularly women, who must exercise caution during pregnancy to avoid sensitization. Sensitization and isoimmunization can occur when the fetal blood mixes with the mother’s blood during the following events:
- Antepartum haemorrhage (APH).
- Ectopic pregnancy.
- Intrauterine death.
- Invasive obstetric testing (e.g. amniocentesis or chorionic villus sampling).
- Miscarriage.
- Termination of pregnancy (abortion).
- Delivery (normal, instrument or caesarean section).
How can rhesus isoimmunisation lead to miscarriages/ foetal demise ?
Most times, when the mother is RhD-ve , the baby is usually Rh D+ve if the father of the baby is Rh D+ve because the baby inherits the rhesus antigen of the father. The antibodies produced by the mother’s body attacks the next baby , not the baby that caused the isoimmunisation to occur because most times those ones are lucky and get away. The red blood cells of the next baby are attacked and haemolysis occurs because the immune antibodies are immunoglobulin G (IgG) in nature and hence, can cross the placenta as said earlier. The rate of haemolysis supersedes that of erythropoiesis in the child and this causes anaemia which eventually leads to miscarriages and intrauterine foetal death.
How can this be prevented?
There are 3 levels of prevention:
- Primary prevention: this involves education of people about rhesus isoimmunisation and the dangers, it also involves encouraging women and everyone to know their blood group.
- Secondary prevention: involves screening pregnant women that come for antenatal care visit. At antenatal visits the woman’s blood should be typed for ABO blood type and the presence or absence of RhD and the quantity should be noted.
If she is negative for RhD protein expression and has not formed anti-D already, she is a candidate for Rho(D) immune globulin (RhIG) prophylaxis to prevent alloimmunization.
3. Tertiary prevention: this involves treating the complications of the disease when there has been exposure to a sanitizing event. If she is positive for anti-D antibodies, the pregnancy will be followed with monthly titres (levels) of the antibody and a quantitative test is performed to determine a more precise amount of foetal blood to which the mother has been exposed. The Kleihauer–Betke test or Flow Cytometry on a maternal blood sample are the most common ways to determine this, and the appropriate dose of Rho(D) immune globulin (RhIG) is calculated based on this information.