Now, British and international researchers say they have finally cracked the case, revealing a previously unknown blood group system that could change how some patients are tested, monitored and treated.
What a blood group really is
Most people only ever hear about A, B, AB and O, sometimes with a plus or minus tagged on the end. That is only the surface of blood typing.
A blood group is a way of classifying red blood cells based on the tiny molecules that sit on their surface. These molecules are called antigens. They can be made of proteins, sugars or combinations of both.
Our immune system constantly scans these antigens. It uses them to decide what belongs in the body and what looks foreign and threatening.
When the “wrong” blood is given in a transfusion, the immune system may attack those unfamiliar antigens, destroying the donated red cells and putting the patient at serious risk.
The familiar ABO and Rh (Rhesus) systems are only two among many. More than 300 recognised blood group antigens have been described worldwide, arranged into dozens of systems. Each person carries a unique combination shaped largely by their genes, with some variants more common in certain regions or ethnic groups.
Beyond A, B and O: the hidden landscape of rare blood
For most hospital patients, knowing their ABO and Rh type is enough to give safe blood. But for a small fraction of people, that is not nearly precise enough.
Specialist blood services, such as NHS Blood and Transplant (NHSBT) in the UK, track rare blood types that occur in fewer than 4 in 1,000 people. These rare types matter when a patient needs repeated transfusions, complex surgery, or care during high-risk pregnancies.
- Common systems: ABO, Rh
- Known rare systems: Bombay, Duffy, Diego, Lewis, MNS, YT and others
- Global tally: more than 380 blood group systems described so far
These unusual patterns often appear “by surprise”. A pregnant woman might show unexpected antibodies on routine screening. A patient might have an unexplained reaction to a transfusion, even though the blood was correctly matched for ABO and Rh. That is usually the moment when a reference laboratory is called in.
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Rare types are not evenly spread across the globe. A blood feature that is exceptional in Europe may be frequent in parts of Africa or Asia, and the reverse is also true. For example, having Rh-negative blood is relatively common in Europeans but uncommon in East Asian populations.
Rarity depends heavily on geography and ancestry, which is why diverse blood donation pools are critical for safe care.
The quiet clue: a mysterious antigen called AnWj
The story behind the new MAL blood group starts with an obscure antigen known as AnWj. Earlier studies suggested that around 99% of people worldwide have AnWj on their red cells.
That leaves a tiny minority – roughly 1% – who do not. For some of them, the absence of AnWj seems to be linked to serious illness, such as certain cancers or blood disorders. In others, including several members of the same family, the pattern looked inherited rather than disease–driven.
The trail goes back to the early 1970s. In 1972, a pregnant woman was rushed to hospital because her unborn baby was in trouble. The baby’s red blood cells were being attacked by antibodies from the mother’s immune system, a condition known as haemolytic disease of the fetus and newborn.
Clinicians realised that the target was AnWj. The mother’s body recognised AnWj as “foreign”, because she lacked that antigen herself. That pregnancy ended tragically, but the case left a crucial question hanging: why did some healthy people have no AnWj at all?
Fifty years of detective work in the genes
Answering that question took decades of gradual progress in genetics and laboratory techniques. A turning point came with large-scale DNA sequencing, which allowed teams at NHSBT and collaborating centres to scan the genes of AnWj-negative individuals in fine detail.
Researchers looked specifically at the sections of DNA that code for proteins present on red blood cells. They were hunting for mutations that could explain the missing antigen.
They eventually traced the absence of AnWj to deletions – missing stretches of DNA – in a gene called MAL, which directs the production of a protein embedded in cell membranes.
When the MAL gene is disrupted, the protein is not made correctly or is not made at all. In those people, AnWj is absent from the surface of their red cells. That tight link between MAL and the presence of AnWj met the criteria for a new blood group system under international rules.
Introducing the MAL blood group system
On that basis, specialists have now recognised MAL as a separate blood group system. People whose red cells lack AnWj due to changes in the MAL gene effectively carry the MAL-negative type.
For most of these individuals, daily life is completely normal. They may never know they are different. The risk appears when they are exposed to AnWj-positive blood.
If a MAL-negative person receives AnWj-positive red cells, their immune system can form potent antibodies against AnWj. A later transfusion, or a future pregnancy carrying an AnWj-positive fetus, can then trigger a violent immune response.
This reaction can damage red blood cells, strain the heart and kidneys, and in severe cases become life-threatening without rapid treatment.
The recognition of MAL as a blood group means laboratories can now design precise tests to identify MAL-negative donors and patients in advance, rather than finding out only after a dangerous reaction.
Why this matters for transfusions
For transfusion specialists, the MAL system adds another piece to an already complex puzzle. Matching donors and recipients now includes the possibility of checking for MAL status when the clinical history suggests a rare incompatibility.
Patients with known rare antibodies are often registered in national or international databases. When they need surgery, childbirth care or emergency treatment, blood services can search globally for compatible donors. MAL-negative individuals are likely to be added to those key registries.
In practice, this can mean:
- Extra screening for patients with unexplained transfusion reactions
- Targeted genetic testing for families with a history of severe pregnancy complications linked to red cell antibodies
- Careful selection and storage of MAL-negative donor units for high-risk cases
Pregnancy, risk and the role of MAL
Pregnancy is one of the most sensitive situations for uncommon blood groups. A mother’s immune system can react to antigens on her baby’s red cells that she herself does not have. The classic case is Rh disease, but MAL now joins the list of possible culprits.
When a MAL-negative woman carries a MAL-positive (AnWj-positive) fetus, her immune system can sometimes create antibodies that cross the placenta and attack fetal red cells. The result is anaemia, jaundice and, in severe cases, organ failure or stillbirth.
Now that MAL has been defined, specialist units can look specifically for anti-AnWj antibodies in complex pregnancies. Early detection allows closer monitoring, potential intrauterine transfusions and carefully planned delivery in a centre with the right blood products on hand.
What MAL tells us about genetics and ancestry
The MAL story also underlines how our blood is shaped by our DNA and family history. Variants in the MAL gene, like other rare blood traits, are more likely to cluster in certain populations or lineages.
For countries with diverse communities, such as the UK, France or the US, that has direct consequences. Blood services need donors from many different backgrounds to make sure that rare combinations are available when a patient needs them.
| Aspect | Common types | Rare types (including MAL) |
|---|---|---|
| Frequency | Seen in large parts of the population | Sometimes fewer than 4 in 1,000 people |
| Impact on routine transfusions | Well covered by standard blood stocks | Requires special matching and tracking |
| Genetic background | Broadly distributed across many groups | Often linked to specific ancestries or families |
Key terms patients often ask about
For anyone who has faced a complicated transfusion or a worrying pregnancy result, the jargon can feel overwhelming. A few concepts help make sense of conversations with doctors:
- Antigen: a marker on the surface of cells that the immune system can recognise.
- Antibody: a protein made by the immune system that attaches to a specific antigen. In blood transfusion, some antibodies can destroy red cells.
- Genotyping: testing someone’s DNA to see which variants of certain genes they carry, including blood group genes like MAL.
- Rare donor: a person whose blood lacks one or more common antigens, making it highly valuable for matching patients with unusual antibodies.
How this might affect real patients
Imagine a patient who has needed many transfusions for a chronic blood disorder. Over time, their immune system has seen a range of donor antigens and formed several rare antibodies, including one against AnWj. Without recognising the MAL system, staff might struggle to explain repeated reactions despite “correct” matching.
With MAL formally defined, laboratories can now label that patient as MAL-negative with anti-AnWj antibodies. Future transfusions can be planned with only MAL-negative units, cutting the risk of another dangerous episode.
A different scenario: a woman whose previous pregnancies ended with severe jaundice in her newborns, for reasons that were never fully clear. Retesting her blood today might show antibodies against AnWj. In a future pregnancy, her care team could watch closely, check the baby’s blood type and plan early intervention if signs of anaemia appear.
These examples show why a discovery made in a research lab can eventually change day-to-day decisions on hospital wards, even if the patients concerned will never hear the term “MAL system” at all.
Originally posted 2026-03-05 01:53:27.