Chapter 2 Answers to self-check questions

Human Blood Group Antigens

2.1 How does the International Society of Blood Transfusion classify Blood Group systems? How does it classify a blood group antigen? Do the number of antigens per system indicate its clinical significance?

The ISBT classifies a blood group system based on a gene that defines the system, and the molecular basis is known. In some instances (e.g. RH and MNS systems) there are more than one gene in the system – Rh=RHD and RHCE; MNS= GYPA, GYPB and GYPE.

 

2.2

  • How does the erythrocyte survive the extreme shear stresses imposed on it during its 120 day life cycle?
  • What are the major proteins of the erythrocyte membrane and its submembranous skeleton?
  • Apart from O2 and CO2 transport what other tasks do erythrocytes serve?

Answers:

  • The erythrocyte membrane skeleton imparts extreme flexibility (termed deformability) to allow the cell to negotiated microcapillaries smaller than its diameter.
  • The major membrane proteins are numerically band 3 (anion exchange protein) approx. 1.1 million per red cell; glycophorin A (approx. 800,000 to 1 million per red cell and glycophorin C (approx. 200,000 per red cell). There are around 100,000 copies of the red cell Rh complex. The major membrane skeletal components include a and b-spectrin, Ankyrin, p55, protein 4.1R and actin. These are complexed with a number of other proteins to give the membrane skeleton its extreme flexibility.
  • Red cells also transport urea, immune complexes, form interactions with other cellular components, process inactive pro-hormones to activate them, remove excess chemokines from the circulation, and prevent red cell destruction by complement.

 

2.3

  • Why is the D antigen so immunogenic?
  • What drives the huge variability in RH genetics?
  • What is the operational definition of partial and weak D and D-elute?

Answers:

  • Because the D-negative polymorphism is the complete absence of a protein (RhD) from the erythrocyte membrane. This offers more immune stimulus than the absence of just one domain (epitope) there are multiple D epitopes known.
  • The genomic arrangement of the RHD-RHCE genes probably accounts for this. They are arranged in a tail-tail configuration and may thus lead to more frequent genomic interactions leading to variants.
  • Partial D individuals can make anti-D when exposed to normal D-positive erythrocytes. Weak and D-elute individuals generally do not, but there are exceptions, some weak D individuals have been shown to make anti-D but this is rare. These include weak D types 1 , 2, 4.2.2 and  15

 

2.4

  • What are the key “back up” systems erythrocytes have in place to maintain water and urea balance?
  • How are membrane transporters attached to the erythrocyte membrane skeleton?
  • Why are some null phenotypes extremely rare (e.g. Co (a-b-); Jk(a-b-))?

Answers:

  • Red cells have “back up” water transporters – AQP3, and also functions as a “back up” urea transporter.
  • Band 3 maintains interactions with the membrane skeletal components Ankyrin and protein 4.1R; the Rh complex with Ankyrin.
  • These null phenotypes are thought to be rare as the erythrocyte membrane components perform the critical roles of water transporters (CO) and urea transporter (JK). Although “back up” systems exist – AQP-3 can transport both water and urea.

 

2.5

  • Why are Kell system antigens depressed in the McLeod phenotype and Kx antigens elevated in the Kell null phenotype?
  • Evolution has driven the emergence of rare blood group phenotypes in parts of the world- why is this and what are their genetic bases?

Answers:

  • The Kx protein is absent in the McLeod phenotype, and this forms a complex via a disulphide bond with the Kell glycoprotein, and thus depresses the insertion of the Kell protein in the red cell membrane.
  • The emergence of the Fy(a-b-) phenotype in West Africa is the best known example. Fy(a-b-) red cells are resistant to malarial invasion. In other parts of the world Gerbich variants are relatively common, yet extremely rare elsewhere they are also resistant to Malarial invasion.

 

2.6

  • Why are PNHIII cells prone to complement mediated lysis?
  • Which blood group system are polymorphic forms of complement components?

Answers:

  • PNHIII individuals have defects in the PIG-A gene which catalyzes the addition of a protein backbone to glycophosphotidylinositol tails. As a consequence they lack the key complement control proteins CD55 and CD59 from their erythrocytes and therefore are prone to complement attack.
  • Chido-Rodgers (CH/RG) polymorphic forms of C4d.

 

2.7

  • What drives the specificity of the PCR?
  • When is blood group genotyping preferred over serological phenotyping?

Answers:

  • The oligonucleotide primers drive the specificity coupled to the annealing temperature used to avoid mispriming from other regions of the genome.
  • When serology is unable to define a variant (e.g. in the RH system), or when blood is difficult or dangerous to obtain e.g. fetal blood in pregnancy

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