Article 013: (The Three Faces of RhD) Full RHD Deletion, Weak D, and Partial D

The Three Faces of RhD: Full RHD Deletion, Weak D, and Partial D

For most people, the Rh blood group seems simple. You are either Rh-positive or Rh-negative. That is how blood types are normally presented: O positive, O negative, A positive, A negative, and so on. But underneath that familiar plus or minus sign is one of the most complicated genetic systems found in human blood.

Modern genetics has shown that the D antigen is not simply an on-and-off switch. Some people have a conventional RHD gene and express the D antigen strongly. Some people have no functional D antigen because the entire RHD gene is absent. Others possess an RHD gene but produce substantially reduced amounts of D antigen, creating one of the situations described as weak D. Still others produce an altered RhD protein in which portions of the normal antigenic structure are different or absent, producing what is known as partial D.

Even these descriptions do not capture the entire story. Scientists have identified many different RHD alleles, and another important phenotype called DEL can express such extraordinarily small amounts of D antigen that routine blood typing may classify the person's cells as RhD-negative. In other words, the familiar positive or negative result is extremely useful clinically, but it can hide considerable genetic diversity underneath.

These differences are not merely scientific curiosities. They can influence whether someone is classified as Rh-positive or Rh-negative, how donated blood is labeled, whether someone should receive Rh-positive or Rh-negative red cells, whether a person could develop anti-D antibodies, and how certain pregnancies are managed. It took decades of observation, experimentation, improved antibody reagents, and eventually molecular genetics to understand what was actually happening.

It Started Before Anyone Knew There Was an RHD Gene

The story begins with the discovery of the Rh blood group itself. In 1939, physicians Philip Levine and Rufus Stetson described an unusual antibody associated with pregnancy and transfusion. The following year, Karl Landsteiner and Alexander Wiener reported another red-cell antibody following experiments involving rhesus monkey cells. The historical relationship between those early antibodies and what eventually became known as the human Rh system turned out to be more complicated than researchers initially believed, but this work helped open an entirely new area of blood-group research.

Researchers soon recognized the extraordinary clinical importance of the D antigen. Today the Rh system is one of the most important blood-group systems in transfusion medicine. The D antigen is particularly significant because a person who lacks D can, under appropriate circumstances, produce anti-D after exposure to D-positive red cells.

This discovery eventually helped medicine understand a devastating problem involving pregnancy. A person lacking D could become immunized to D carried by fetal red cells and subsequently produce antibodies capable of crossing the placenta and destroying D-positive fetal red blood cells. But before researchers completely understood this process, they encountered another mystery: some people's blood simply did not behave like ordinary D-positive or D-negative blood.

Read the history and clinical review of weak D and RhD

The Strange Blood Samples That Didn't Behave Normally

During the 1940s, researchers began encountering red cells that reacted unusually weakly with Rh typing reagents. Alexander Wiener described inherited weakly reacting Rh factors during this early period, but one of the most important observations came from British hematologist Frederick Stratton.

In 1946, Stratton, working with the Blood Transfusion Service at Manchester Royal Infirmary, investigated blood whose Rh typing produced contradictory reactions. Some anti-D sera reacted with the red cells while other sera produced much weaker reactions or none at all. This was important because a straightforward positive-or-negative model could not easily explain why different anti-D preparations behaved differently toward the same person's cells.

Stratton investigated the inheritance of the characteristic and proposed that he had identified a new Rh variant. He called it Dᵘ, a term that would remain part of transfusion medicine for decades. His report, A New Rh Allelomorph, appeared in Nature in 1946.

At the time, scientists could observe these reactions and follow their inheritance through families, but they could not examine the RHD gene itself. DNA sequencing was decades away. Researchers knew they were seeing different expressions of D without yet knowing the molecular changes responsible.

Read Stratton's original Nature report

From Dᵘ to Weak D

For decades, Dᵘ remained common terminology. As laboratory technology improved, however, scientists realized that the old category contained biologically different phenomena. The terminology gradually shifted toward the concept of a serologic weak D phenotype, describing red cells that react weakly or require particular testing conditions to demonstrate D.

One useful introductory way of understanding classic weak D is to imagine a conventional D-positive red cell displaying many copies of the D antigen across its surface. A weak-D cell may display substantially fewer D antigen sites. A test that easily recognizes a conventional D-positive sample may therefore produce a much weaker reaction with those cells.

That explanation is useful for someone first learning about RhD, but modern genetics requires an important qualification. Weak D is not one mutation and should not be thought of as one uniform biological condition. Numerous RHD alleles can produce weak serologic expression, and their molecular and clinical behavior is not necessarily identical.

This distinction has become so important that modern transfusion literature separates the serologic weak D phenotype, which describes what happens during laboratory antibody testing, from a molecularly defined weak D type, which describes the person's RHD genotype. A person can therefore first be identified as having a serologic weak D phenotype and then undergo RHD genotyping to determine which molecular variant is responsible.

Genetics Finally Explained Why

The major breakthrough came when scientists moved beyond observing reactions in test tubes and began examining the genes responsible for Rh proteins. We now know that the Rh system is primarily associated with two closely related genes on chromosome 1. RHD encodes the RhD protein responsible for D, while RHCE encodes the closely related RhCE protein responsible for combinations of the C, c, E, and e antigens.

Molecular research during the 1990s dramatically changed our understanding of weak D. Instead of merely observing that someone's cells reacted weakly with anti-D, researchers could investigate the RHD gene and determine exactly which genetic variant was present.

Scientists discovered numerous molecular weak-D types. Many classic weak-D variants involve amino-acid substitutions in portions of the RhD protein located within the red-cell membrane or toward the interior of the cell. Such changes can interfere with how efficiently RhD protein is incorporated into the membrane, reducing the amount of D antigen displayed at the surface.

But this is precisely where the simple explanation needs refinement. Saying that every weak-D person has an entirely normal D antigen but simply "less of it" is too broad. There are numerous molecular weak-D alleles, and the distinctions between weak D and partial D are not perfectly captured by one simple quantity-versus-quality rule. Molecular classification provides much more information than the strength of an agglutination reaction alone.

Partial D: When the Antigen Itself Is Different

Partial D introduces a different problem. A person's cells can express substantial D antigen and still not carry every antigenic feature associated with conventional D.

The RhD protein presents multiple antigenic structures that antibodies can recognize. These recognizable structures are commonly described as epitopes. With partial D, genetic changes alter the RhD protein so that one or more D epitopes are absent or significantly changed.

A useful analogy is to imagine conventional D as a complex key with many different ridges. Classic weak D can be pictured as displaying fewer copies of that key. Partial D is different: the key itself has been altered, so one or more of its normal ridges are missing or changed.

This difference can have important consequences. A person's immune system recognizes the D structures they possess as self. But if that person is exposed to conventional D-positive cells containing a D epitope that their own red cells lack, that unfamiliar structure can potentially be recognized as foreign. Some people with partial D can therefore form alloanti-D following exposure to conventional D-positive red cells.

This is one of the reasons that simply saying someone has "some D" is not enough to resolve every transfusion question. The molecular variant can matter.

Then There Is Complete RHD Gene Deletion

At another end of the RhD story is a fundamentally different genetic situation. Many people who are conventionally D-negative have no RHD gene on the chromosome where it would otherwise occur. When a person inherits RHD-deleted haplotypes such that no functional RHD gene is present, conventional RhD protein is not produced and D antigen is absent from the red-cell surface.

Whole RHD gene deletion is an especially important cause of the D-negative phenotype in populations of European ancestry. But human populations differ substantially in the molecular causes underlying a D-negative laboratory result.

That means RhD-negative does not universally equal complete RHD deletion. Some people who type D-negative possess RHD genetic material but carry an altered or nonfunctional allele. Other individuals possess hybrid genes involving RHD and RHCE. Still others belong to the extremely interesting category known as DEL.

This population diversity is one reason molecular genetics has changed transfusion medicine so dramatically. Two people can both have "Rh-negative" written on a laboratory report while the molecular explanation underlying that result is completely different.

DEL: When D Is Almost Invisible

DEL adds another layer to the story.

The DEL phenotype was first described in 1984 by Okubo and colleagues at the Osaka Red Cross Blood Center. They encountered red cells that appeared D-negative even when additional testing for weak D was performed. Yet after the cells were exposed to anti-D, researchers could recover the antibody through an adsorption-elution procedure. This demonstrated that extraordinarily small amounts of D antigen were actually present.

The phenotype became known as Del, from the idea of D detected through eluate, with modern literature commonly using DEL terminology in discussing the associated molecular variants.

DEL expression can be extraordinarily low. Conventional serologic testing may therefore report these cells as D-negative even though D antigen is present. Historically, specialized adsorption-elution testing could reveal it; today, molecular testing can identify the RHD alleles responsible.

This makes DEL conceptually fascinating. A conventional D-positive person displays readily detectable D antigen. A weak-D person may express reduced amounts that can still be demonstrated using appropriate serologic methods. A DEL individual can express so little D that conventional serologic tests, including methods historically used to detect weak D, may fail to demonstrate it.

DEL Also Shows Why Ancestry Matters

DEL is especially important because its prevalence varies dramatically between populations.

In East Asian populations, a substantial proportion of people who appear D-negative by routine serology actually possess a DEL allele. One particularly important variant is RHD*DEL1, also called Asian-type DEL or RHD:c.1227G>A.

Reviews have reported that DEL phenotypes occur among substantial proportions of serologically D-negative people in China, Japan, and Korea, whereas DEL is much less common among D-negative populations of European ancestry. This means that the molecular explanation behind a seemingly identical "D-negative" laboratory result can differ substantially depending on population background.

The genetics of DEL have also turned out to be highly diverse. By 2023, the International Society of Blood Transfusion database listed dozens of RHD alleles associated with DEL phenotypes. Molecular mechanisms include splice-site variants, missense changes, hybrid alleles, frameshift variants, premature termination codons, and deletions involving portions of RHD.

So DEL itself, just like weak D, is not one single genetic condition.

DEL Has Clinical Importance Too

DEL is not simply an interesting laboratory phenomenon.

If red cells from a DEL donor are incorrectly considered truly D-negative, they can potentially be transfused to a genuinely D-negative recipient. Cases of anti-D immunization following transfusion of DEL red cells have been reported, which is one reason identifying certain DEL donors can matter for blood-bank safety.

At the same time, modern evidence shows why it is dangerous to treat every DEL allele as clinically identical. Recent work concerning Asian-type DEL, RHD*DEL1, supports managing individuals with this particular genotype as D-positive in several clinical circumstances. A 2024 review described evidence supporting D-positive transfusion strategies and no requirement for routine anti-D prophylaxis in pregnancy for individuals specifically shown to have Asian-type DEL. That conclusion applies to the defined genotype and should not automatically be generalized to every DEL variant.

This is another example of the central lesson of modern RhD genetics: the exact allele matters.

So Are These Really the Three Types of Rh-Negative?

Not exactly, and this distinction is worth making very clear.

Full RHD deletion, weak D, and partial D should not scientifically be described as the only three kinds of Rh-negative blood. Weak D and partial D are categories of RhD variation, and DEL provides an especially clear example of why the system cannot be reduced to three genetic possibilities.

DEL, hybrid RHD/RHCE genes, nonfunctional RHD alleles, and numerous individual weak and partial D variants demonstrate that the complete Rh system is substantially more complicated.

For someone first learning about RhD, however, full deletion, weak D, and partial D provide an excellent starting framework. DEL can then show why even that improved model has exceptions.

Why Someone Can Get Conflicting Rh Results

This biology helps explain something that has confused patients and blood donors for decades. A person can occasionally receive different RhD results from different laboratories or testing methods, and that does not automatically mean somebody made a mistake.

Traditional blood typing relies on antibodies that recognize structures on the surface of red blood cells. Different anti-D reagents and laboratory platforms can have different sensitivities and recognition characteristics. A weak D antigen might produce a weak or negative reaction with one testing method but become detectable with another.

Partial D adds another possibility because different anti-D reagents may recognize different D epitopes. DEL goes even further because the amount of D antigen can be below the detection threshold of routine serologic testing altogether.

What once looked like confusing laboratory behavior can now sometimes be explained at the DNA level.

Why Blood Donors and Patients Have Historically Been Treated Differently

Blood banks have historically had good reason to approach weak or uncertain D differently depending on whether the person being tested was donating blood or receiving it.

If a donor expresses D but is incorrectly labeled D-negative, those red cells might be transfused into a truly D-negative recipient. If sufficient D antigen is present, that recipient could potentially become immunized and develop anti-D.

For a patient receiving blood, the concern works in the opposite direction. When D status is uncertain, treating the recipient conservatively as D-negative can avoid exposing someone capable of producing anti-D to conventional D-positive cells.

These competing concerns helped produce different historical testing strategies for donors and recipients. Molecular testing now offers a way to resolve some of that ambiguity by determining which RHD allele the person actually carries.

Pregnancy Is Where These Differences Can Become Especially Important

RhD became widely known outside transfusion laboratories largely because of pregnancy.

A genuinely D-negative pregnant person carrying a D-positive fetus can be exposed to fetal red cells. The immune system may recognize D as foreign and produce anti-D. Those antibodies can cross the placenta and destroy D-positive fetal red cells, causing hemolytic disease of the fetus and newborn.

The development of Rh immune globulin, or RhIG, transformed prevention of this disease.

RhD variants complicated the seemingly simple positive-versus-negative model. If a pregnant person's cells produce a weak or unusual D reaction, the clinical question is not merely whether any D can be detected. The question can become which RHD genotype is present and whether that genotype carries a meaningful risk of alloanti-D formation.

Not Every Weak D Person Has the Same Clinical Risk

"Weak D" is not one single mutation. Molecular testing has identified numerous weak-D types, and they should not automatically be assumed to have identical clinical implications.

The best-established example involves weak D types 1, 2, and 3. AABB/CAP recommendations have supported managing people with these molecularly defined types as RhD-positive because available evidence indicates they are not considered at risk for forming alloanti-D following conventional D exposure in the circumstances evaluated.

Other weak-D types are different. Some have documented anti-D formation, while evidence for others remains limited. Historically, precautionary management has therefore been recommended for variants for which sufficient evidence was unavailable.

This is why someone being told simply "you have weak D" does not necessarily have the complete answer.

Often the next question is which weak D type?

Serology Shows the Phenotype. Genetics Can Reveal the Cause.

Traditional blood typing remains extraordinarily useful. It is fast, well established, and answers the questions needed for the overwhelming majority of routine blood typing.

But serology primarily tells us how a person's red cells react with particular antibodies.

Genotyping asks a different question. It examines the DNA responsible for producing those red-cell structures.

That distinction gives us two useful terms. Phenotype describes the observable characteristic, such as how strongly D is expressed or detected. Genotype describes the underlying genetic variant.

Two people can therefore have similar-looking serologic results while possessing different RHD genotypes. Conversely, people carrying related molecular variants can sometimes produce different-looking laboratory reactions depending on the reagent and method being used.

That is why modern RhD investigation increasingly combines the century-old power of serology with molecular genetics when an unusual result actually matters clinically.

A Better Way to Picture the RhD Spectrum

For someone new to this subject, it may help to imagine RhD as a spectrum rather than simply a light switch.

At one end is conventional D-positive, where D antigen is readily expressed and detected.

Then there are weak-D phenotypes, where D expression can be reduced, although numerous different genetic variants exist underneath that general laboratory description.

There are partial-D phenotypes, in which the structure of D differs and particular epitopes may be absent or altered.

There is DEL, where D expression can be so extraordinarily low that routine testing may classify the cells as D-negative.

And there is true absence of conventional D, which can result from complete deletion of RHD or from other genetic mechanisms that prevent functional D expression.

Even this spectrum is an educational simplification. Nature does not organize every RHD allele into perfectly separated boxes.

That complexity is precisely what makes the Rh blood-group system so fascinating.

What We Know Today

After more than eight decades of Rh research, one conclusion has become unavoidable: RhD biology is much more complicated than the plus or minus sign printed beside a person's ABO blood group.

A conventional D-positive individual generally expresses readily detectable RhD protein. A person with complete RHD deletion may produce no RhD protein from that absent gene. A person with weak D can possess one of numerous RHD variants associated with reduced or altered D expression. A person with partial D possesses an RhD protein in which part of the conventional antigenic structure is changed. A person with DEL can express such tiny quantities of D that routine serology may classify the cells as D-negative even though molecular and specialized testing demonstrates that D is present.

Beyond these categories are hybrid genes, nonfunctional alleles, population-specific variants, and additional molecular arrangements that researchers continue to characterize. The importance of these differences is not merely academic. They can influence blood typing, transfusion decisions, anti-D formation, pregnancy management, RhIG decisions, blood-donor classification, and the interpretation of apparently contradictory laboratory results.

What began in the 1940s as scientists noticing that certain blood samples behaved strangely with anti-D eventually became a story written in DNA. Early researchers could observe unusual reactions and inheritance patterns, but they could not see the RHD sequence responsible.

Modern researchers can.

And that has changed our understanding of RhD from a simple positive-or-negative label into what it really is, a remarkably diverse human genetic and antigen system hiding behind one tiny + or − printed beside our blood type.

Sources and Further Reading

1. Sandler, Chen & Flegel: “Serological weak D phenotypes: A review and guidance for interpreting the RhD blood type using the RHD genotype.”
   This is probably the best overall source in the article. It covers the history of weak D, Stratton, Dᵘ terminology, weak D versus partial D, DEL, RHD genotyping, transfusion, pregnancy, and AABB/CAP recommendations.
   Read the full article free on PubMed Central
2. Frederick Stratton: “A New Rh Allelomorph,” Nature, 1946.
   This is the actual historical paper in which Stratton described the unusual inherited D variant that became known as Dᵘ. It's especially valuable because we're not relying only on somebody else's retelling of the discovery.
   Read Stratton's 1946 Nature paper
3. Wagner et al: “Molecular Basis of Weak D Phenotypes,” Blood, 1999.
   This is one of the most important molecular papers behind the genetics section. The researchers sequenced the RHD exons of weak-D samples and identified numerous molecular weak-D types, helping explain why weak D occurs at the genetic/protein level.
   Read the Blood journal paper
4. Wagner & Flegel: Research on complete RHD gene deletion.
   This work concerns the molecular mechanism responsible for the common RHD gene deletion associated with the D-negative phenotype, particularly relevant to the full-deletion portion of our article.
   Read the Blood journal article on RHD deletion
5. Review of RhD variants and DEL phenotypes.
   This is useful for understanding why the familiar positive/negative distinction doesn't capture all RhD biology, including people whose D expression is too low for ordinary serologic detection.
   Read the DEL and RhD variant review on PubMed Central
6. Evidence and guidance concerning RHD genotyping and pregnancy.
   This covers why identifying the actual RHD genotype can matter when a pregnant patient has a serologic weak-D phenotype and discusses the implications for Rh immune globulin.
   Read the PubMed record
7. Additional modern research on DEL genetics.
   This is useful for explaining that DEL itself isn't a single mutation and that numerous RHD alleles can produce extremely low D expression.
   Read the DEL genetics review on PubMed Central