ABO Blood Group Antigens

Blood Group Antigens

• The ABO blood type is determined by the presence or absence of specific antigens (A and/or B) on the surface of red blood cells (RBCs).
• Genes coding for these antigens are on chromosome 9 and inherited from both parents.
• The ABO gene encodes glycosyltransferase, an enzyme that adds sugar molecules to the surface of RBCs.
• The A allele of glycosyltransferase adds N-acetyl galactosamine, creating the A antigen.
• The B allele adds galactose, creating the B antigen.
• The O allele variant does not modify the glycoprotein, so no A or B antigen is created.
• A and B antigens are complex carbohydrates known as glycoproteins, made of a protein backbone with branched sugar molecules.
• The sugar molecules determine the blood type.
• Each allele codes for a glycosyltransferase enzyme which carries a different sugar to the RBC.
• The added sugar determines the antigen.
• The O allele does not create a functional glycosyltransferase enzyme.

Inheritance Patterns

• The A and B alleles show codominance, meaning both alleles are expressed when present together.
• For example, an AB genotype has both A and B antigens on their RBCs, expressing both alleles equally.
• A and B alleles are codominant while the O allele is recessive to both A and B alleles.
• The O allele does not express antigens
• In an AO genotype, the A antigen is expressed as the A allele is dominant over the O allele.

###Agglutination and Haemolysis

• The body produces antibodies against antigens that are not present on ones own cells.
• If blood types are mixed, antibodies bind to foreign antigens on the donor blood causing agglutination and haemolysis.

ABO Blood Group System

• Type A blood has A antigens and anti-B antibodies.
• Type B blood has B antigens and anti-A antibodies.
• Type AB blood has both A and B antigens but no antibodies, therefore it can receive all blood types.
• Type O blood lacks A and B antigens but has both anti-A and anti-B antibodies, therefore it is the universal donor.

Rhesus (Rh) Factor

• An important antigen on the surface of RBCs, if present, the blood is Rh+; if absent, the blood is Rh-.
• Anti-Rh antibodies (anti-D) are only produced after exposure to the Rh antigen.
• E.g., An Rh- person is given blood from an Rh+ donor and then the Rh- recipient will produce antibodies.

Frequency of Blood Types in Australia

• O+ is the most common blood type at 40% of the population.
• O- 9%
• A+ is the second most common at 31%.
• A- 7%
• B+ 8%
• Then B- 2%, followed by AB+ at 2%, and AB- 1%.
• Over 80% of the Australian population is Rh+.

Blood Group Compatibility

• This depends on creating antibodies against all antigens that the recipent does not poses.
• To simplify, imagine a "Game of Thrones" analogy, if "House A+" were to donate blood it would be "declaring war" (by donation) on every other house (blood type) other than itself.
• Refer to the compatibility chart in the document for more specific compatibility.

Haemolytic Disease of the Newborn (HDN)

• Reported in more than 20,000 babies a year in the USA alone.
• HDN is caused by antibodies from a pregnant woman cross the placenta and attack the red blood cells of her fetus.
• Anti-Rhesus antibodies have become standard in treatment for 50+ years.
• HDN occurs when an Rh- mother carries an Rh+ foetus.
• Women who are Rh- receive the Anti-D during pregnancy and within 72 hours of giving birth, which neutralises the Rh+ antigen
• Normally at 28 and 34 weeks’ gestation (as an antenatal prophylaxis). Note women with Rh+ do NOT require anti-D.
• Injection is normally given slowly by deep intramuscular injection
• Additional doses can also be given earlier in the pregnancy (ie: patient has a history of premature births)
• a sensitizing event" occurs (examples next slide), then anti-D is also recommended to be given at that time

Sensitising Events Can Cause HDN

• Occurs when an Rh- mother carries an Rh+ foetus.
• During childbirth or other events that lead to mixing of maternal and foetal blood from a sensitizing event, the mother can become sensitised to Rh+ RBCs.
• Sensitising events include:
  • Ectopic pregnancy: Fertilised egg is implanted outside of the womb, commonly in a fallopian tube
  • Termination of pregnancy, including miscarriage.
  • Ultrasound-guided procedures, such as: fetoscopy, amniocentesis ,chorionic villus sampling , cordocentesis.
  • Abdominal trauma that causes uterine activity, and or abdominal pain
  • Antepartum haemorrhage: Genital bleeding during the late stages of pregnancy.
  • External cephalic version: Turning the fetus from breech positions into a head-down position for labour.
• For an Rh- female, the anti-D immunoglobulin should be given prior to these procedures, or as soon as possible

HDN If Sensitisation Has Occurred

• Women who are Rh+ do NOT receive the Rh(D) Immunoglobulin
• (ie: a Rh- woman has developed anti-D antibodies) a variety of steps are taken
• Special care and attention is provided to the mother and foetus, under the supervision of an obstetrician.
• Assessments are made early-on regarding the concentration of the antibodies.
• The foetus is managed to the best of the local health care’s requirements
• Foetal transfusions may be required if the foetus risks becoming anaemic, etc..
• In many areas of the world, appropriate healthcare is not available for this kind of occurrence.
• All of Australia’s anti-D plasma comes from a tiny pool of around 200 donors.
• Hydrops fetalis and haemolytic disease of the newborn also leads to anemia in the new born, in turn leading to enlarged spleen and liver