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
Structure:
- Introduction
- ABO system: Ag, Ab
- Rhesus system: Ag, Ab
- O-ve
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 | Properties |
|---|---|
| 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 | Properties |
|---|---|
| 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 Red Cells
| Factor | Properties |
|---|---|
| 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 |
| 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 |