Cis AB

Cis AB is a rare blood group phenotype that occurs within the ABO blood group system. It is characterized by the unique presence of both A and B antigens on the same allele. Unlike the more common AB phenotype, where an individual inherits one A allele from one parent and one B allele from the other, individuals with the Cis AB phenotype inherit a single allele that expresses both A and B antigens. This condition is a result of a genetic mutation or a gene fusion event, leading to the production of both antigens by a single gene.

Genetics

The ABO blood group system is determined by the ABO gene located on chromosome 9. The ABO gene encodes an enzyme that modifies the H antigen into either A or B antigen. In the case of the Cis AB phenotype, the gene mutation allows for the creation of an enzyme capable of adding both N-acetylgalactosamine (the A antigen) and galactose (the B antigen) to the H antigen. This results in red blood cells that express both A and B antigens simultaneously.

Inheritance

Cis AB inheritance is autosomal codominant. This means that an individual only needs to inherit the mutated allele from one parent to express the phenotype. The other parent can contribute an O allele, or an A, B, or even another Cis AB allele, which would not alter the Cis AB expression. The inheritance pattern makes the Cis AB phenotype less predictable and more complex than the standard ABO inheritance.

Prevalence

The Cis AB phenotype is extremely rare, with the highest prevalence reported in certain East Asian populations. However, it remains a rarity worldwide, contributing to the complexity and fascination surrounding this unique blood type.

Clinical Significance

The clinical significance of the Cis AB blood type lies primarily in transfusion medicine. Individuals with the Cis AB phenotype can receive red blood cells from A, B, AB, and O donors, making them universal recipients like individuals with the AB phenotype. However, their plasma contains antibodies against neither A nor B antigens, which is a unique feature compared to the standard AB phenotype. This characteristic must be carefully considered during blood transfusions and organ transplants to prevent adverse reactions.

Diagnosis

Diagnosis of the Cis AB phenotype is typically made through blood typing tests that reveal unexpected reactions indicative of both A and B antigens on the same allele. Further genetic testing can confirm the presence of the Cis AB allele.

Conclusion

The Cis AB blood group phenotype is a fascinating example of genetic variation within the ABO blood group system. Its rarity and unique characteristics pose interesting challenges and considerations for medical practice, particularly in transfusion and transplant medicine. Understanding and recognizing this rare phenotype is crucial for the safe and effective management of patients requiring blood products.

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