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

List:

· 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:

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:

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:

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