Question 9(p.2)

Explain the ABO blood groups and how blood group is determined. How is blood tested for compatibility using the ABO system?

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College Answer

The main points candidates were expected to cover included a detailed discussion of the ABO
antigens, the evolution of IgM antibodies to these antigens and the prevalence of blood
groups in the general population. Answers tabulating blood groups against expected antigens
and antibodies present as well as agglutination reactions to anti-sera were most effective.
Discussion of the saline agglutination test was essential and extra marks were awarded for mention of the Coombs test. Candidates were required to demonstrate an understanding of
cross-matching, specifically the testing of donor red blood cells against recipient serum.
Syllabus J1 2.a
References: Review of Medical Physiology (Chapter 27), 22nd ed.
Ganong, pp. 537-539.


  • Blood group is defined by the antigens found on the surface of red cells
    • Numerous antigen molecules exist, of which A, B and the Rhesus antigens are the most common
    • A and B are carbohydrate (oligosaccharide) molecules on the glycocalyx of red cells
    • The presence of the antigen identifies the blood group, i.e. the patient with the AB blood group has A and B antigens on the surface of their red cells
    • Antibodies (IgM) to the other antigens usually develop in the first months of life
    • O, A, B and AB are phenotypes. 
    • An "O" phenotype indicates that the red cells have neither A nor B antigens.
      Group  Antigens  Antibodies Can receive blood from:  Can donate blood to:
      A A Anti-B A and O A and AB
      B B Anti-A B and O B and AB
      AB A and B None Everybody AB
      O None

      Anti-A and Anti-B

      Only O Everybody
  • The presence of blood groups in the general population:
    • O group: 49% of the population
    • A group: 38% of the population
    • B group: 10% of the population
    • AB group: 3% of the population
    • 85% of the population is Rh-positive
  • Laboratory determination of ABO grouping:
    • Forward grouping: patient RBCS mixed with known anti-A and anti-B antibodies
    • Reverse grouping:  patient's plasma is mixed with known Type A or Type B red cells
    • Both must agree to formally identify the blood group
  • Compatibility testing:
    • Determines compatibility for tranfusion by directly testing donor and recipient samples
    • Saline agglutination: looks for anti-ABO IgM in the blood of the recipient.
      • Immediate-spin crossmatch: patient serum and donor red cells are centrifguged for 10 minutes and checked for agglutination
    • Indirect antiglobulin test: looks for clinically significant IgG antibodies.
      • Patient's plasma is  incubated together with the donor red cells


Pourazar, Abbasali. "Red cell antigens: Structure and function." Asian journal of transfusion science 1.1 (2007): 24.

Daniels, G. "Structure and function of red cell surface antigens." ISBT Science Series 1.1 (2006): 3-8.

Pawlak, Z., and M. Lopez. "Development of antigens ABH and Ii in children between 0 to 16 years old." Revue francaise de transfusion et immuno-hematologie 22.3 (1979): 253-263.

Berg, J.M., Tymoczko, J.L. & Stryer, L. (2007). Biochemistry, 6th Ed. (p. 315). New York, NY: W.H. Freeman


British Committee for Standards in Haematology, et al. "Guidelines for pre‐transfusion compatibility procedures in blood transfusion laboratories." Transfusion Medicine 23.1 (2013): 3-35.