Which red blood cell (RBC) antigen, if positive, would rule out the Bombay phenotype?

Differentiating Bombay from Para-Bombay Phenotype

The I antigen, if positive on red blood cell testing, would definitively rule out the Bombay phenotype. 1

Understanding the Bombay and Para-Bombay Phenotypes

The Bombay (Oh) and para-Bombay phenotypes are rare blood group variants characterized by defects in H antigen expression:

• Bombay phenotype (Oh):

  • Complete absence of H, A, and B antigens on red blood cells
  • Caused by homozygous mutations in the FUT1 gene
  • RBCs test negative for I antigen
  • Serum contains anti-H, anti-A, and anti-B antibodies

• Para-Bombay phenotype:

  • Weak or partial expression of H antigen on red blood cells
  • May have weak A or B antigen expression (e.g., "Ah para-Bombay")
  • RBCs test positive for I antigen
  • Associated with defects in FUT1 and/or FUT2 genes

Key Differentiating Antigens

Among the options presented (I, P, Lea, Leb), the I antigen is the critical differentiator:

1. I antigen (Option A):

   • Present in para-Bombay phenotype
   • Absent in true Bombay phenotype
   • A positive I antigen test would definitively rule out Bombay phenotype 1

2. P antigen (Option B):

   • Not specifically associated with differentiating Bombay from para-Bombay
   • Present in both phenotypes

3. Lea antigen (Option C):

   • Not a reliable differentiator between Bombay and para-Bombay
   • Expression depends on Lewis gene status, not H antigen status

4. Leb antigen (Option D):

   • Requires H antigen for expression
   • Absent in both Bombay and para-Bombay phenotypes

Clinical Significance

Correct identification of Bombay versus para-Bombay phenotype is critical for:

• Transfusion safety: Patients with Bombay phenotype can only receive Bombay (Oh) blood, which is extremely rare (1 in 10,000 in India, 1 in 250,000 in Caucasians) 2
• Preventing fatal hemolytic transfusion reactions: Misdiagnosis can lead to incompatible transfusions 3
• Blood bank protocols: Both forward and reverse typing with appropriate controls are essential for identification 2

Laboratory Diagnosis

To differentiate between Bombay and para-Bombay:

• RBC antigen testing for H, A, B, and I antigens
• Serum antibody testing for anti-H, anti-A, and anti-B
• Saliva testing for secretor status
• Molecular testing of FUT1 and FUT2 genes

Conclusion

When evaluating a patient with suspected Bombay or para-Bombay phenotype, testing for the I antigen provides the most reliable differentiation between these two rare blood group variants. A positive I antigen result would rule out the Bombay phenotype and confirm para-Bombay phenotype.