2021A Question 3
Outline the physiology of blood groupings that allows O negative packed red cells to be safely transfused to most patients.
Examiner Report
68.1% of candidates achieved a pass in this question.
The major domains assessed in this question were:
- Physiology of ABO and Rhesus grouping
- Physiology of Incompatible transfusion reaction
- Explanation of why O neg blood is safe
Credit was given for other relevant correct material.
Common problems were:
- Defining Rh status by antibody rather than antigen presence/absence
- Confusion over IgM and IgG antibodies, which blood group (ABO or Rhesus) they pertain to and which cross the placenta
- Using vague terminology to describe antigen/antibody reactions without specifying whether the antigen is in the donor or recipient and vice versa
- Describing group AB as presence of the “AB” antigen rather than the presence of A and B antigens.
- Confusion over which Rh group can develop Rh antibodies and how these develop
- Lack of detail on why absence of an antigen/antigens equates with safety
- Only discussing one part of the question to the detriment of the other part - for example only discussing physiology of blood groups and not why O neg is safe or only discussing why O neg is safe not discussing physiology of blood grouping
Model Answer
Structure:
- Introduction
- ABO system: Ag, Ab
- Rhesus system: Ag, Ab
- O negative red cells
Introduction
- RBC membranes express antigens (Ag)
- Antibodies (Ab) are produced against non-self RBC Ag following exposure
- Transfusion safe only if recipient won’t produce Ab against donor Ag
ABO System
| Factor | Property |
|---|---|
| Antigens | - Carbohydrate Ag - Also present in diet - Common oligosaccharide foundation = “O antigen” - Type A adds N-acetylgalactosamine - Type B adds galactose - Type AB adds both |
| Antibodies | - Non-self anti-A, Anti-B developed in first year of life if type O blood (diet exposure) - (Anti-O = Bombay is rare) - More pentameric IgM - More antigenic - Doesn’t cross placenta - Incompatibility → Intravascular haemolysis (worse) - Note no plasma in donor packed RBCs, hence donor’s antibodies unimportant |
Rhesus System
| Factor | Property |
|---|---|
| Antigen | - Protein Ag - Many types (A, a, B, b etc); D most important - Non-self Anti-RhD only developed after exposure to RhD+ blood (no diet exposure) |
| Antibodies | - More monomeric IgG - Less antigenic - Incompatibility → Extravascular haemolysis (not quite as bad) - Does cross placenta (hydrops foetalis) - Important to avoid sensitization in females of childbearing potential: - First time: Rh- mother, Rh+ neonate → Sensitization - Second time: Rh+ neonate, maternal Anti-RhD IgG causes hydrops foetalis, haemolytic Dx of newborn |
O Negative
| Factor | Property |
|---|---|
| Structure | - RBCs: “O Ag” but no A, B or RhD Ag - Plasma has anti-A, anti-B, +/- anti-RhD but these are washed from RBC unit |
| Universality | - Safe for A,B,O whether D+ or D- - Useful in emergency when no time for crossmatch |
| Exceptions | - Bombay group: Anti-O antibody - Other groups: Kell, Kidd, Duffy - Importance variable - May cause reaction if sensitised |