Blood groups

Question 1

ABO blood group system

Answer 1

• ABO locus is on Chromosome 9, expression also requires H(FUT1) gene on Chromosome 19
• A+B antigens are indirect gene products, the direct products are transferase enzymes
• A+B antigens are co-dominant, whereas O is recessive
• Red cells are produced with a Type-2 short chain carbohydrate attached to the red cell surface
  + A gene = n-acetyl galactosamine + H-antigen
  + B gene = Galactose + H-antigen
  + AB = Competition between A+B transferases results in an A+B antigen formation

Question 2

A antigen subgroups

Answer 2

• The efficiency with which A transferase adds it sugar is also genetically controlled, giving rise to subgroups of A
• 80% individuals are A(1), 20% are A2, other subgroups are very rare (A3, AX, AM)

Question 3

Acquired B antigen

Answer 3

• Blood group A can be charged to group B in some disease conditions ie bacterial enzymes released into circulation causing deacetylation of the n-acteyl galactosamine into galactose
• Often reverts/resolves as the condition improves
• Occures in (ie) Rectal + Bowel carcinoma, Perforation/Ulceration, Infection/Septicaemia

Question 4

Oh Phenotype

Answer 4

• Also known as Bombay Phenotype
• Rare individuals are phenotypicall hh, but no H substance is produced due to inability to add fucose to the Type 2 chain
• Causes some individuals to be group O despite having A/B alleles
• These indiviuals have naturally occuring anti-H Ab, making them incompatible with all ABO blood types except Oh which is extremely rare

Question 5

Rh blood group system

Answer 5

• Rh locus is found on Chromosome 1
• Comprises of 2 highly homologous closely linked genes RhD + RhCE
• Antigens are direct gene products, each gene codes for a separate transmembrane protein that forms group D, C, c, E, e
• RhD codes for D-antigen
• Haplotypes: R1, R2, R0, R3, r’, r’’, r, ry

Question 6

Rhnull blood type

Answer 6

• Where rare individuals fail to produce both RhD + RhCE proteins
• 2 forms: Amorph + Regulator type
• Rhnull cells are morphologically + functionally abnormal: Stomatocytosis + Increased cation transport respectively
• Affected individuals have chronic haemolytic anaemia

Question 7

Kell system

Answer 7

• KEL locus are found on Chromosome 7
• Antithetical allels K/k + they are co-dominant
• Kell protein is a single pass glycoprotein + has strcutural homology with a famjily of enzymes termed as zinc-neutral endopeptidases
• Expression depends on production of Kx protein by genes at XK locus on X chromosome