Subgroups of (Blood Type) B?

This post brought to you by...the Letter B!
Yes, they do exist! We are generally much more familiar with subgroups of A, especially in patients who have made an Anti-A1, but subgroups of B do exist in patient populations. They are rare, but this lack of prevalence can likely be partially explained by Blood group B having a much lower prevalence than A to begin with. 

Here's a breakdown of the B subgroups:

B3 Subgroup

Antigenic Expression:
  • The B3 subgroup presents with a weaker expression of the B antigen when compared to the regular B blood group. This diminished strength is often the result of changes in the sugar structures that form the basis of the B antigen.
Mixed-Field Agglutination:
  • B3 cells often display a phenomenon called mixed-field agglutination when tested with anti-B serums. Mixed-field agglutination refers to the simultaneous presence of agglutinated and non-agglutinated red cells in the same sample. This can occur if only a fraction of the RBCs in the sample express the B antigen, leading to partial clumping.
Genetic Inheritance:
  • The B3 phenotype appears to have an autosomal dominant pattern of inheritance. It's inherited in families, and if one parent carries the gene, there's a 50% chance that their offspring will also express this subgroup.
Differentiation from Acquired B Antigen:
  • Acquired B antigen often arises due to external factors, typically associated with diseases affecting the gastrointestinal tract. The enzymes produced by certain bacteria can modify the A antigen to resemble B antigen, leading to the "acquired B" phenotype.
  • Differentiating between B3 and acquired B is essential for several reasons:
    • Transfusion Reactions: Patients with acquired B can experience transfusion reactions if given blood from true B or AB donors. Knowing the difference helps guide appropriate blood product selection.
    • Organ Transplantation: Similar to transfusions, ensuring the correct blood type and its nuances are critical for organ transplants.
    • Diagnostic Significance: Acquired B antigen can be an indicator of an underlying disease. Recognizing this can be a diagnostic clue, especially in patients with unexplained anemia or those who have a history suggestive of gastrointestinal disease.

Further Considerations:

  1. Testing: Enhanced testing methodologies, including the use of different antisera or molecular techniques, may help clarify ambiguous results.
  2. Clinical Implications: While B3 is primarily of academic interest, its recognition can prevent potential transfusion-related complications. Blood bank specialists must be aware of its existence and its differentiation from the more clinically significant acquired B phenotype.

In summary, the B3 subgroup, while rare, represents a fascinating intersection of genetics, biochemistry, and clinical medicine within the realm of immunohematology.

Bx (Bend) Subgroup

Antigenic Expression:
  • The Bx subgroup, similar to B3, displays a weaker expression of the B antigen on the surface of red blood cells. However, what distinguishes Bx (Bend) cells from typical B cells is their unique reactivity pattern.
Reactivity with Anti-B:
  • The Bx cells demonstrate weak agglutination (clumping) when reacted with anti-B sera. This weak agglutination can sometimes be misinterpreted, leading to potential misclassification of the blood type, especially if other subgroups or acquired conditions aren't considered.
Temperature-Dependent Reactivity:
  • One of the defining features of the Bx (Bend) subgroup is its enhanced reactivity at colder temperatures, typically around 4°C. When Bx cells are tested at this lower temperature, the agglutination reaction with anti-B sera is stronger.
  • The temperature-dependent reactivity suggests specific structural or conformational changes in the B antigen that favor binding to the anti-B antibody at lower temperatures.
Clinical Implications:
  • Blood Transfusion: Recognizing the Bx (Bend) subgroup is critical for blood banks, as misclassification can lead to transfusion reactions. If a person with the Bx phenotype is mistyped as group O and then receives B or AB blood, it could cause a reaction.
  • Laboratory Practices: Because of its temperature-dependent reactivity, blood banks must be vigilant when performing tests, especially if the reactions are carried out at different temperatures. Moreover, if a blood bank specialist encounters unexpected weak B reactions, the Bx (Bend) subgroup should be considered, and additional tests at various temperatures may be conducted.
  • Genetics and Biochemistry: While the Bx (Bend) subgroup has been defined serologically, the genetic and biochemical underpinnings are not as well-understood as the main blood group types. There may be genetic mutations or alterations in the enzymes involved in synthesizing the B antigen, leading to the distinct Bx phenotype.

Bm (B modified) Subgroup

Antigenic Expression:

  • The Bm subgroup carries a modified or variant form of the B antigen on the surface of red blood cells. This modification leads to a differential reactivity when exposed to certain anti-B reagents.

Reactivity with Anti-B:

  • Bm cells typically show weaker reactions or sometimes even negative reactions with specific monoclonal anti-B reagents. This can create confusion in blood typing, potentially leading to misclassification.
  • The reactivity pattern varies depending on the origin of the anti-B reagent used, especially between monoclonal and polyclonal sources. Polyclonal anti-B sera, derived from multiple immune cells, might still show agglutination with Bm cells, while certain monoclonal reagents, originating from a single immune cell clone, might not.

Geographical and Ethnic Distribution:

  • The Bm phenotype has been found in various populations worldwide but remains relatively rare. Its presence in both African and Caucasian populations suggests that it's not limited to a specific ethnic group, though its prevalence might vary across different communities.
  • Genetics and Biochemistry: The specific genetic and biochemical basis for the Bm phenotype is not entirely clear. However, it is believed to arise from genetic mutations or alterations in the enzymes responsible for B antigen synthesis, leading to a modified structure.

Bel Subgroup

Antigenic Expression:

  • Individuals with the Bel subgroup have red blood cells that express an extremely low amount of the B antigen—so low that it's nearly indistinguishable from an O blood type using conventional serological methods.
  • Because of this minute expression, routine ABO typing may detect these cells as type O, leading to potential misclassification.

Anti-B Production:

  • Despite the almost non-existent B antigen expression, individuals with the Bel phenotype can produce anti-B antibodies. The production of this antibody suggests that the level of B antigen present on their cells is insufficient to induce tolerance.
  • The anti-B produced by Bel individuals is not benign. If a Bel individual receives blood from a B or AB donor, this anti-B can target and destroy the transfused red blood cells, leading to a hemolytic transfusion reaction—a serious and potentially life-threatening adverse event.

Clinical Implications:

  • Blood Transfusion: Given the potential for Bel individuals to produce anti-B, it's paramount to identify this subgroup accurately. Misidentification could lead to providing B or AB blood to a Bel recipient, which would trigger a hemolytic reaction.
  • Laboratory Challenges: The primary challenge is the potential misclassification of Bel as O due to the almost non-existent B antigen expression. Advanced serological methods, such as adsorption and elution techniques, or molecular tests may be required to detect and confirm the presence of the Bel phenotype.
  • Pregnancy and Hemolytic Disease of the Newborn (HDN): While the focus is often on transfusion, it's also essential to consider potential complications in pregnancy. If a Bel mother is pregnant with a fetus expressing the B antigen, there's a risk, albeit low, for HDN.

Historical Context:

  • The name "Bel" is derived from the initial patient's name in whom this phenotype was first identified. Over time, as with many blood group anomalies, the name has been used to represent the entire subgroup.

Acquired B Blood Group Phenotype

Nature of Change:

  • As the name suggests, this alteration in blood group is not inherited. Instead, it arises due to external factors during an individual's lifetime.
  • The phenomenon involves a modification of the A antigen on the red cell surface, causing it to mimic the B antigen.


  • The transformation of the A antigen to resemble the B antigen is primarily attributed to the action of bacterial enzymes. Certain bacteria, especially those flourishing in gastrointestinal conditions, produce enzymes known as deacetylases.
  • These microbial deacetylase enzymes remove acetyl groups from the A antigen, altering its structure. The modified A antigen then mimics the B antigen in serological reactions.

Clinical Implications:

  • Blood Typing Discrepancies: The most immediate consequence of the Acquired B phenomenon is a discrepancy in blood typing. An individual previously typed as group A may appear as group AB due to the presence of the modified A antigen that reacts with anti-B sera.
  • Transfusion Concerns: Given the altered blood typing results, there's a potential risk of transfusion errors. However, it's important to note that the acquired B antigen typically does not trigger an immune response. Thus, even if group B blood was transfused into a patient with Acquired B, hemolytic reactions are unlikely. Still, it's crucial to adhere to blood transfusion guidelines and provide group-specific or type O blood when in doubt.

Associated Conditions:

  • Acquired B is most frequently associated with gastrointestinal diseases, where the growth and activity of certain bacteria are enhanced. Common conditions include:
    • Colon cancer
    • Intestinal obstruction
    • Peptic ulcers
    • Other gastrointestinal malignancies
  • The association with gastrointestinal diseases, especially colon cancer, implies that the presence of Acquired B could serve as a diagnostic hint, suggesting the need for further gastrointestinal evaluation.

Resolution and Diagnosis:

  • Acquired B is generally a transient phenomenon. Once the underlying gastrointestinal condition is addressed or if the microbial flora is altered (e.g., due to antibiotics), the blood group typically reverts to its original type.
  • If Acquired B is suspected, repeating the blood group test after treating the individual with acid or enzymes can revert the modified A antigen back to its original state, thus confirming the diagnosis.

Differentiation from Genuine AB:

  • In order to differentiate Acquired B from a true AB blood type, a detailed patient history is crucial. Salivary blood grouping, which remains unaffected by the acquired B phenomenon, can also aid in distinguishing between the two.

In essence, Acquired B serves as a testament to the dynamic nature of the human body and its interactions with the microbial environment. Recognizing such anomalies and understanding their underlying mechanisms ensures patient safety and can also provide valuable diagnostic insights.

B(A) Subgroup of Blood Group B

Nature and Origin:

  • The B(A) phenotype exhibits characteristics of both B and A antigens on the red cell surface, but the A-like properties are weaker than those seen in a true A antigen.
  • The origin of the A-like quality in these B cells isn't entirely clear. It's believed that this phenotype arises due to the activity of an A transferase enzyme that is functioning at a reduced capacity. This enzyme adds specific sugars to the H antigen, turning it into an A antigen. However, in the case of B(A) subgroup, this enzyme's activity isn't as efficient as in regular A blood group individuals, leading to a weaker A expression.

Serological Characteristics:

  • Blood samples from individuals with B(A) typically react with anti-B and anti-A sera, but the reaction with anti-A is weaker. This can cause discrepancies in blood grouping.
  • Monoclonal anti-A reagents might not detect the weaker A antigen, but polyclonal anti-A reagents (which are generally more sensitive) may produce a reaction.

Clinical Implications:

  • Blood Typing Confusion: The most significant concern with B(A) is the potential for misclassification. If not detected and classified correctly, a B(A) individual could be mistyped as AB or B.
  • Transfusion Issues: A person with a B(A) phenotype can produce anti-A antibodies, even though they have A-like properties on their cells. This means they can potentially have a transfusion reaction if given blood from a true A or AB donor.


  • Thorough serological tests, often employing a series of anti-A reagents and adsorption-elution techniques, can help in correctly identifying this subgroup. Adsorption techniques can remove the interfering antibodies, and elution can then be used to identify the eluted antibodies.
  • A detailed patient history is also beneficial. Some B(A) phenotypes are acquired due to underlying conditions, similar to the Acquired B phenomenon.


  • The prevalence of the B(A) subgroup varies among populations but is generally rare.
  • Some studies suggest a higher occurrence in certain Asian populations, though it's still a rare phenomenon.


Post a Comment