By Suherman Candra Maholtra 
For over five decades, the genetic origin of a specific marker on human red blood cells remained one of the most enduring enigmas in transfusion medicine. Since its initial discovery in 1972, the AnWj antigen has presented a significant challenge to hematologists and researchers worldwide. However, a landmark study led by scientists at NHS Blood and Transplant (NHSBT) in Bristol, in collaboration with the International Blood Group Reference Laboratory (IBGRL) and the University of Bristol, has finally identified the genetic source of this marker. By tracing the antigen to the MAL gene, researchers have officially established MAL as a new human blood group system, providing a definitive molecular framework for identifying rare individuals who lack this antigen and protecting them from life-threatening transfusion reactions.
The resolution of the AnWj mystery is more than a technical victory; it represents a profound advancement in our understanding of human biology. While the ABO and Rh systems are the most widely recognized blood groups, they are merely the tip of a vast biological iceberg. Red blood cells are coated with a complex array of proteins and sugars that serve as antigens—biological flags that the immune system uses to distinguish "self" from "non-self." When a patient receives blood that carries an antigen their own body lacks, their immune system may mount a devastating attack on the foreign cells. For the tiny fraction of the population that is AnWj-negative, this discovery is a literal lifesaver.
The Historical Context of Blood Group Discovery
The history of blood group identification began in 1900 with Karl Landsteiner’s discovery of the ABO system, which earned him the Nobel Prize. This was followed by the discovery of the Rh system in 1940. Since then, the catalog of human blood groups has expanded slowly but steadily. For an antigen to be recognized as a distinct blood group system by the International Society of Blood Transfusion (ISBT), scientists must prove it is genetically unique and identify the specific protein that carries it.
The AnWj antigen was first identified in 1972 in a patient whose blood reacted strangely during cross-matching. Despite its discovery over 50 years ago, the protein responsible for the AnWj marker remained elusive. The difficulty lay in the extreme rarity of the inherited AnWj-negative phenotype. While more than 99.9% of the global population is AnWj-positive, those who are negative fall into two categories: those who have suppressed the antigen due to illness, and those who are born without it due to a rare genetic mutation. Because so few people possess the inherited form, researchers lacked the sufficient genetic material required for a definitive study until the advent of modern genomic sequencing.
The Genetic Breakthrough: Identifying the MAL Gene
The research team utilized whole exome sequencing, a powerful genetic tool that focuses specifically on the protein-coding regions of the genome. By analyzing the DNA of a small cohort of inherited AnWj-negative individuals—including a family from an Arab Israeli community and the original patient identified in the 1970s—the researchers looked for shared genetic anomalies.
The analysis pointed directly to the MAL gene, which is responsible for producing the Mal protein (Myelin and Lymphocyte protein). This was an unexpected find, as the Mal protein is a small, highly hydrophobic membrane protein that had not previously been associated with red blood cell antigens. The researchers found that individuals with the inherited AnWj-negative phenotype possessed homozygous deletions in the MAL gene. In genetic terms, this means they lacked a functional copy of the gene from both parents, resulting in the total absence of the Mal protein on the surface of their red blood cells.
To confirm their findings, the team employed sophisticated gene-editing techniques. By introducing a functional MAL gene into laboratory-grown cells that were previously AnWj-negative, they were able to make those cells express the AnWj antigen. Conversely, when they introduced the mutated version of the gene, the cells remained AnWj-negative. This "cause and effect" demonstration provided the final proof required to link the MAL gene to the AnWj antigen.
Clinical Implications and Transfusion Safety
The clinical significance of this discovery cannot be overstated. When an AnWj-negative individual is exposed to AnWj-positive blood—either through transfusion or pregnancy—they can develop potent antibodies. If they receive a subsequent transfusion of positive blood, these antibodies can cause a hemolytic transfusion reaction, where the immune system rapidly destroys the donor’s red blood cells. This can lead to kidney failure, systemic shock, and death.
By identifying the MAL gene, scientists can now develop highly accurate genotyping tests. Previously, identifying AnWj-negative donors required rare and expensive antisera (antibodies) derived from the blood of sensitized patients. Now, blood banks can use DNA-based screening to scan large donor registries for the specific MAL mutations. This will significantly increase the availability of compatible blood for rare patients.
The discovery also helps clinicians distinguish between inherited AnWj deficiency and "acquired" deficiency. In many cases, patients with certain hematological cancers or disorders stop expressing the AnWj antigen temporarily. Understanding the genetic baseline allows doctors to determine if a patient’s negative status is a permanent genetic trait or a symptom of an underlying disease, which dictates very different approaches to clinical management.
Case Studies: Real-World Applications in 2026
The practical necessity of this research was highlighted by two complex medical cases reported in 2026. In one instance, a 75-year-old man suffering from severe anemia was found to have an anti-AnWj autoantibody. Because no compatible blood was available, doctors had to make the difficult decision to proceed with an unmatched transfusion. Genomic testing later confirmed his MAL gene was normal, suggesting his condition was acquired. Fortunately, he did not suffer a severe reaction, but the case underscored the terrifying "blind spot" clinicians face when dealing with unknown antigens.
In a second case involving a patient with high-grade B-cell lymphoma, researchers documented the first use of the drug sutimlimab to treat AnWj-associated hemolysis. The patient’s immune system was aggressively attacking transfused blood, but by using a drug that blocks the complement pathway of the immune system, doctors were able to stabilize the patient’s condition. These cases demonstrate that while the MAL blood group system is rare, its impact on individual patient outcomes is critical.
Official Recognition and the Expanding Map of Human Blood
The International Society of Blood Transfusion (ISBT) formally ratified MAL as the 47th blood group system, assigning it the designation ISBT 047. This classification was part of a rapid period of discovery in hematology. Between 2024 and 2026, four new systems were added to the official register: ER, CD36, ATP11C, and MAL. By late 2026, the discovery of the JAMA system brought the total number of recognized blood group systems to 49.
This acceleration in discovery is largely due to the falling costs of genomic sequencing and the increased collaboration between international blood reference laboratories. As Louise Tilley, Senior Research Scientist at IBGRL, noted, the resolution of the AnWj mystery was the culmination of 20 years of her personal research and 50 years of collective effort. The complexity of the Mal protein—being small and deeply embedded in the cell membrane—made it one of the most difficult puzzles the field has ever solved.
Chronology of the MAL Blood Group Discovery
- 1972: The AnWj antigen is first discovered, but its protein carrier and genetic origin remain unknown.
- 1970s–2010s: Sporadic cases of AnWj-negative individuals are identified globally, but the rarity of the phenotype prevents large-scale genetic study.
- 2015: A crucial blood sample is donated by the original patient from the 1970s, providing a vital link for future genomic testing.
- 2021–2023: Researchers at NHSBT and the University of Bristol apply whole exome sequencing to samples from five genetically AnWj-negative individuals.
- 2024: The MAL gene is identified as the source of the AnWj antigen. Laboratory experiments using gene editing confirm the link.
- 2025: The findings are peer-reviewed and presented to the international hematology community.
- 2026: The ISBT formally recognizes MAL as the 47th blood group system. Genotyping for MAL begins to be integrated into rare donor screening platforms.
Expert Reactions and Future Outlook
The scientific community has reacted with enthusiasm to the news. Professor Ash Toye, Director of the NIHR Blood and Transplant Research Unit at the University of Bristol, emphasized the role of modern biotechnology in solving "outstanding puzzles" of human biology. He noted that the ability to manipulate gene expression in developing blood cells was the "smoking gun" that allowed the team to prove MAL’s role beyond a doubt.
Nicole Thornton, Head of IBGRL Red Cell Reference, highlighted the global impact of the discovery. "We are passionate about making these discoveries for the benefit of rare patients around the world," she stated. The transition of AnWj from a "serological curiosity" to a "genetically defined system" allows for a new era of personalized transfusion medicine.
Looking forward, the identification of MAL serves as a blueprint for resolving the remaining "orphan" antigens in the human blood system. There are still several markers known to exist on red blood cells that have not yet been linked to a specific gene. As genomic tools become more accessible, the map of human blood will continue to grow, ensuring that even the rarest patients can receive safe, compatible care. For the 0.1% of people whose lives depend on these discoveries, the end of the 50-year mystery of AnWj is nothing short of a medical miracle.
