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


  • Introduction
  • ABO system: Ag, Ab
  • Rhesus system: Ag, Ab
  • O negative red cells


  • 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

- Carbohydrate Ag

- Also present in diet

- Common oligosaccharide foundation = “O antigen”

- Type A adds N-acetylgalactosamine

- Type B adds galactose

- Type AB adds both


- 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

- 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)


- 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

- 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


- Safe for A,B,O whether D+ or D-

- Useful in emergency when no time for crossmatch


- Bombay group: Anti-O antibody

- Other groups: Kell, Kidd, Duffy

- Importance variable

- May cause reaction if sensitised

Last updated 2021-08-23

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