2021A03 Outline the physiology of blood groupings that allows O negative packed red cells
to be safely transfused to most patients.



·        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:


·   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:


·   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:


·   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



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