A 28-year-old Indian woman with AB blood group is scheduled for elective surgery and needs blood transfusion. The inheritance pattern marked **A** in the diagram shows that both her IA and IB alleles are fully and independently expressed, resulting in both A and B antigens on her red blood cell surface. Which of the following best explains why this woman can receive red blood cells from donors of any ABO blood group?

Explanation

## Why "She lacks both anti-A and anti-B isoagglutinins in her serum, making her a universal RBC recipient" is right The codominant expression of both IA and IB alleles (marked **A** in the diagram) results in the AB phenotype with both A and B antigens fully expressed on RBC surfaces. Critically, individuals with AB blood group do NOT develop naturally occurring IgM isoagglutinins (anti-A or anti-B antibodies) because their immune system recognizes both A and B antigens as "self" during infancy. Therefore, AB individuals can receive RBCs from donors of any ABO type (A, B, AB, or O) without risk of hemolysis, making them universal RBC recipients. This is a direct consequence of the codominant expression pattern shown at **A**: both alleles are fully functional and expressed, preventing the development of antibodies against either antigen. [Robbins Pathology 10e Ch 14; Harrison 21e Ch 110] ## Why each distractor is wrong - **She has both A and B antigens, which neutralize any donor antibodies through antigen-antibody complexation**: While AB individuals do have both antigens, the mechanism of universal recipiency is NOT antigen-antibody neutralization in circulation. Rather, it is the ABSENCE of anti-A and anti-B antibodies in the first place. Antigen-antibody complexation would still cause hemolysis if antibodies were present. - **Her codominant IA and IB alleles produce glycosyltransferases that modify donor RBC antigens in circulation**: This is mechanistically incorrect. The glycosyltransferases encoded by IA and IB alleles act during RBC development in the bone marrow to add sugars to the H-antigen precursor on the patient's own RBCs. They do not circulate and do not modify donor RBC antigens in vivo. - **She has naturally occurring IgG antibodies against all ABO antigens, which prevent hemolysis of donor cells**: This is false on two counts: (1) AB individuals do NOT develop anti-A or anti-B antibodies at all; (2) isoagglutinins are IgM, not IgG. IgG antibodies would actually cause problems in transfusion (delayed hemolytic transfusion reaction). **High-Yield:** AB blood group = universal RBC recipient (lacks anti-A and anti-B antibodies); O blood group = universal RBC donor (lacks A and B antigens); codominance means BOTH alleles are fully expressed in the heterozygote. [Robbins Pathology 10e Ch 14; Harrison 21e Ch 110]