2016B Question 04

Outline the physiology of blood groupings that allows O negative packed red cells to be safely transfused to most patients.

Examiner Report

75% of candidates achieved a pass in this question.

Main points expected for a pass:

  • Neg status is determined by the ABO and Rhesus blood groups
  • A classification of ABO system according to antigens expressed on the RBCs
  • Recognition that antibodies (Abs) develop in the plasma against non-self ABO antigens (and to delineate these)
  • Classification of the Rhesus system based on the presence or absence of the Rhesus antigen
  • Recognition that ABO incompatibility results when donor RBCs expressing an antigen (A &/or B) which recipient does not possess are transfused. The recipient will have antibodies to the foreign antigen in their plasma, resulting in an immune reaction.
  • Recognition that O Neg blood cells express no A or B antigens and no Rhesus antigen and hence will not provoke an immune response when transfused to most.

Additional credit was gained for discussion of:

  • The ABO system as the most antigenic
  • How the ABO Abs are formed after birth and their nature
  • The Rhesus system having multiple alleles, but the D allele as the most important
  • The need for prior exposure to the D antigen to produce anti-D antibodies (unlike ABO Abs) and the importance of avoiding this sensitization esp in women of childbearing age (hence the use of Rh – blood)
  • Rare blood groups eg the Bombay blood group where O neg blood may cause an immune reaction
  • Other more minor blood antigens which may cause reaction in sensitized patients
  • The importance of removing plasma from the donated RBCs (as O plasma contains both anti-A and anti-B antibodies)
  • The utility of O Neg blood in emergency situations with critical time pressures

The most common error was the description of antibodies against the ABO antigens as “A Abs & B Abs” when they are actually anti-A Abs and Anti-B Abs.

Model Answer


  • Introduction
  • ABO system: Ag, Ab
  • Rhesus system: Ag, Ab
  • O-ve


  • 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 Properties

- 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 Properties

- 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 Red Cells

Factor Properties

- 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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