A longitudinal eight-year study conducted at Siriraj Hospital under Mahidol University in Thailand has led to the discovery of a rare hybrid B(A) blood type. The research, directed by hematologist Janejira Kittivorapart, analyzed over 544,000 blood samples to identify this uncommon variant within the traditional ABO blood group system. The study uncovered four novel genetic mutations responsible for the phenotype, marking the first time these specific genetic variations have been documented in the Thai population. This discovery provides critical data for improving global transfusion safety and resolving complex blood-typing discrepancies.
Understanding the B(A) Phenotype
The B(A) blood type is a rare sub-phenotype that challenges conventional classification methods due to its hybrid characteristics.
Antigenic Profile
In standard serology, individuals inherit genes that code for specific sugar molecules, or antigens, on the surface of their red blood cells. The B(A) phenotype occurs in individuals who are genetically classified as blood type B but concurrently express trace amounts of the A antigen on their red blood cells.
The Mechanism of Dual Expression
The dual expression of antigens happens due to subtle structural changes in the glycosyltransferase enzymes. Normally, the B allele produces an enzyme that attaches galactose to the cell membrane. In B(A) variants, mutations alter the enzyme’s binding site, causing it to occasionally attach N-acetylgalactosamine, the sugar responsible for the A blood type.
Genetic Discoveries and Population Frequency
The eight-year screening project relied on advanced genetic sequencing to identify the underlying cause of the hybrid blood type.
Novel Mutations
Genetic analysis of the Thai subjects revealed four previously undocumented mutations within the ABO gene. These specific nucleotide substitutions alter the amino acid sequence of the B transferase enzyme, increasing its efficiency in synthesizing minor amounts of A antigen alongside regular B antigens.
Statistical Rarity
Out of the 544,000 individual samples screened during the study, only three individuals possessed this specific hybrid phenotype. This places the estimated frequency of the B(A) blood type at approximately 0.00055 percent, which translates to an occurrence rate of roughly one in 180,000 individuals in Thailand.
Clinical Challenges in Transfusion Medicine
The presence of hybrid blood types creates technical obstacles during routine clinical procedures and laboratory diagnostics.
ABO Discrepancies
Standard forward grouping tests detect the weak A antigen expression, causing the sample to mimic the standard AB blood type. However, reverse grouping tests looking for antibodies in the plasma often show a different result because these individuals retain anti-A antibodies in their serum. This contradiction between cell typing and serum typing is known as an ABO discrepancy.
Transfusion Safety Protocols
Misidentifying a B(A) individual can lead to major clinical risks. If a B(A) patient receives standard AB blood, their natural anti-A antibodies can attack the transfused cells, causing an acute hemolytic transfusion reaction. To prevent this, laboratories must use advanced monoclonal antibody screening and genetic sequencing to ensure these patients receive packed red blood cells from pure type B donors.
Comparison of Rare Blood Types and Variants
The table below outlines how the newly analyzed B(A) phenotype compares with other globally recognized rare blood variants.
| Blood Group Variant | Genetic/Antigenic Characteristics | Clinical Transfusion Implication |
| B(A) Phenotype | Genetically type B; possesses mutated B enzyme that synthesizes trace A antigens. | Must receive Type B blood to avoid reactions from anti-A serum antibodies. |
| A(B) Phenotype | Genetically type A; produces minor amounts of B antigen due to mutated A transferase. | Requires Type A blood; misclassification as AB creates transfusion risks. |
| Bombay Blood Group (hh) | Lacks the H antigen entirely; cannot produce A or B antigens regardless of genotype. | Can only receive blood from another Bombay phenotype individual. |
| Rh-Null Blood Type | Totally lacks all 61 antigens in the Rh system; called “Golden Blood.” | Universally compatible within the Rh system but can only receive Rh-null blood. |
IASPOINT Booster Facts for UPSC
- ABO Gene Location: The ABO blood group system is encoded by a single gene locus situated on the long arm of Chromosome 9 ($9q34.2$).
- Karl Landsteiner: The Austrian biologist who discovered the main A, B, and O blood groups in 1900, laying the foundation for modern blood banking and earning the Nobel Prize in Physiology or Medicine in 1930.
- Forward vs. Reverse Grouping: Forward grouping tests a patient’s red blood cells using known commercial antibodies to identify surface antigens. Reverse grouping tests the patient’s serum against known red blood cells to identify circulating antibodies.
- The H Antigen Dilemma: The H antigen is the chemical precursor required to build both A and B antigens. In the rare Bombay phenotype, individuals lack the FUT1 gene needed to make the H antigen, meaning they cannot form A or B antigens even if they carry the functional ABO genes.