What Antigens Are in a Suspect With Type O+ Blood?

A person with type O+ blood carries the H antigen on their red blood cells, the D antigen of the Rh system (which is what the “+” signifies), and a collection of other blood group antigens from systems beyond ABO and Rh. Their red cells lack both the A and B antigens, which is the defining feature of group O, but “O” does not mean the cells are antigen-free. In a forensic context, this antigen profile matters because it determines what can be detected in blood, saliva, and other biological evidence left at a scene.

The H Antigen on Type O Red Cells

A common misunderstanding is that type O blood has no antigens at all. In reality, the surface of every type O red blood cell is covered with H antigen, a sugar structure that serves as the foundation for the entire ABO system. In people with type A or type B blood, enzymes modify H antigen by adding another sugar to it, converting it into A antigen or B antigen. In type O individuals, those enzymes are non-functional, so H antigen remains unmodified and accumulates on the cell surface in large quantities.

The H antigen is built from precursor sugars by a specific enzyme encoded by the H gene, which attaches a fucose sugar to the precursor chain.¹ Because type O individuals do not convert H into anything else, their red cells actually express more H antigen than people with types A, B, or AB. This is not just a theoretical detail. In forensic serology, reagents that detect H antigen can help confirm a type O sample, and the relative strength of the H reaction can even help distinguish O from other groups that have leftover unconverted H.

What the “+” Means and the Rh Antigens Beyond D

The plus sign in O+ refers to the D antigen of the Rh blood group system, which is the most clinically significant Rh antigen. If someone is Rh-positive, their red cells carry the D protein on their surface. But the Rh system is not just D. It also includes the C, c, E, and e antigens, and a type O+ suspect will carry some combination of these as well.

The Rh antigens are carried by specialized transmembrane proteins found only on red blood cells. The RhD protein and the closely related RhCE protein share about 92% of their amino acid sequence and sit embedded in the cell membrane in a similar way.² An O+ individual will be D-positive by definition, and will also express some version of C or c, and E or e, depending on which alleles they inherited. In many populations, the most common Rh phenotype among D-positive people includes C, e, and c antigens alongside D. One large study of blood donors found D present in about 94% of donors, with e antigen in 98%, C in 87%, c in 58%, and E in about 20%.³

Knowing a suspect is O+ tells you the D antigen is present, but it does not tell you which combination of C/c and E/e they carry. Extended Rh typing requires additional testing and can narrow identification further, which is useful in forensic casework when comparing biological evidence to a known suspect.

Antigens From Other Blood Group Systems

Beyond ABO and Rh, every person carries antigens from dozens of additional blood group systems. These are not routinely reported on a blood type card, but they are physically present on the red cell surface and detectable with appropriate reagents. For a type O+ suspect, these additional antigens include members of the Kell, Kidd, Duffy, MNS, and Lewis systems, among others.

The exact profile varies from person to person. In the same donor study mentioned above, the Duffy antigen Fy(a) was found in about 87% of samples, Fy(b) in about 58%, Kidd antigen Jk(a) in roughly 82%, Jk(b) in about 67%, M antigen in about 89%, N in 65%, S in 55%, and s in 89%.³ The Kell antigen (K) was rare at about 3.5%, while its counterpart k was nearly universal. These frequencies shift across populations, but the point is that typing as “O+” is only the beginning of a person’s full antigen fingerprint.

In forensic investigations, extended antigen profiling was historically one of the few ways to distinguish between two people who shared the same ABO and Rh type. DNA profiling has largely replaced this for individual identification, but blood group serology still plays a role in screening and corroboration, particularly in resource-limited settings or when dealing with degraded samples where DNA extraction fails.

Antibodies in Type O+ Plasma

Antigens are only half of the blood group picture. The other half is the antibodies circulating in the plasma, and type O individuals carry a distinctive set. Healthy people naturally produce antibodies against whichever ABO antigens their own red cells lack.⁴ Since type O red cells lack both A and B antigens, a type O person’s plasma contains both anti-A and anti-B antibodies.

These antibodies are not something the body learns from previous blood exposure. They appear in the first months of life, triggered by exposure to bacteria in the gut that carry sugar structures resembling A and B antigens. Research has shown that feeding blood group-active bacteria to people can stimulate antibody production, and that intestinal bacteria are a major driver of natural anti-A and anti-B antibody levels.⁵ This bacterial origin explains why even newborns begin developing these antibodies shortly after their gut flora establishes itself.

For forensic purposes, the presence of both anti-A and anti-B antibodies in a bloodstain or dried serum sample can help confirm a type O classification from the serum side, complementing cell-based antigen testing. This two-pronged approach, testing both antigens and antibodies, reduces the chance of a misclassification.

Secretor Status and Antigens in Body Fluids

Whether a suspect’s blood group antigens show up in saliva, sweat, semen, or tears depends on something called secretor status. Secretors are individuals who release soluble forms of their blood group antigens into body fluids.⁶ Non-secretors do not, and their body fluids will not reveal ABO blood group information through standard serological tests.

The secretor trait is controlled by a separate gene from the one that determines ABO type. The enzyme it encodes is responsible for building the H type 1 structure in secretory tissues, and that structure then serves as the foundation that other enzymes can modify into A or B antigens in secretions.⁷ For a type O secretor, body fluids will contain detectable H antigen but no A or B antigen. For a type O non-secretor, body fluids will lack all three.

Roughly 80% of people are secretors, though rates vary by population. In forensic work, if saliva or another body fluid is recovered from a crime scene, a secretor with type O blood will leave H antigen behind that testing can pick up. A non-secretor suspect, though, would leave a body fluid sample that appears antigen-negative, which can create confusion if investigators expect to find a blood group marker. One study found that among their sample population, blood groups A and O showed 100% secretor status, though this may reflect the specific population studied rather than a universal rule.⁸

How Forensic Antigen Testing Actually Works

In criminal investigations, blood group typing of evidentiary samples traditionally relied on serological methods, adding known antibodies to a sample and watching for a reaction. If anti-A reagent causes red cells to clump, A antigen is present. If anti-B does, B antigen is present. If neither causes clumping, the sample is type O. Add anti-D reagent: if clumping occurs, the sample is Rh-positive.

This approach works well with fresh liquid blood but becomes trickier with dried stains, degraded samples, or body fluids from non-secretors. Bacteria at a crime scene can also interfere. Microbial contamination of biological evidence can cause false results by altering or mimicking blood group antigens, making careful analysis critical in aged or contaminated samples.⁹

Molecular genotyping has increasingly supplemented traditional serology. By testing the DNA itself rather than the protein or sugar it encodes, labs can determine blood group type even from degraded samples. Studies comparing serological phenotyping to DNA-based genotyping have found that genotyping tends to produce fewer false results, particularly in complex cases such as patients who have received transfusions or samples that have been exposed to environmental degradation.¹⁰ For forensic identification purposes, DNA-based STR profiling has largely overtaken blood group typing, but the antigen profile still provides corroborative information and remains useful in mass disaster identification or when DNA is unavailable.

Antigens Beyond Red Blood Cells

Blood group antigens are not confined to red cells. H antigen, for instance, is found on the surface of many tissue types throughout the body, including cells lining the gut, respiratory tract, and urinary tract.¹ This broad tissue distribution is part of why blood group antigens can be detected in secretions and why they were once so valuable in forensic serology before the DNA era.

Separately, platelets carry their own antigen systems, including HLA antigens (shared with other nucleated cells) and platelet-specific HPA antigens. Research cataloging platelet donor banks has documented polymorphism across multiple HPA types, including HPA-1 through HPA-6, HPA-10, HPA-15, and HPA-21.¹¹ White blood cells carry HLA antigens as well. None of these are related to ABO or Rh type, meaning two people who are both O+ could have completely different HLA and HPA profiles. In the forensic context, HLA typing was actually used for individual identification before modern DNA profiling became standard.

When Type O Is Not Really Type O

There are rare situations where someone appears to be type O but genetically is not, or where someone who is genetically O tests as something else. These edge cases matter in both clinical and forensic contexts.

The most famous example is the Bombay phenotype. People with this condition lack functional H antigen entirely because of mutations in the H gene. Without H antigen, they cannot build A or B antigen either, so their red cells look like type O in standard testing. But they are not genetically O; they may carry A or B genes that simply have no H substrate to work with. The distinction matters because Bombay individuals produce anti-H antibodies in addition to anti-A and anti-B, making their serum reactive against ordinary type O blood, which is loaded with H antigen.¹² If a forensic sample from a Bombay individual were typed using standard methods, it would be classified as O. Only molecular testing or specialized serology would reveal the true genotype.

In the other direction, disease or infection can temporarily alter a person’s apparent blood type. In one documented case, a patient with blood group A1 developed what appeared to be a B antigen on their red cells during severe septicemia, along with partial loss of their A1 antigen.¹³ Bacterial enzymes can modify the sugars on red cell surfaces, creating antigen-like structures that mimic other blood groups. For someone who is genuinely type O, the risk runs the other way: bacterial contamination of a crime scene sample could theoretically produce false antigen reactions that make O blood appear to carry A or B markers.

Why Type O Is So Common and What Malaria Has to Do With It

Type O is the most common blood group worldwide, and its global distribution is not random. Research has linked type O to a substantial protective effect against severe malaria. One study found that group O was associated with a 66% reduction in the odds of developing severe malaria compared to non-O groups.¹⁴ The mechanism involves rosetting, a process where infected red blood cells stick to uninfected ones, forming clumps that block small blood vessels. Type O red cells form smaller, weaker rosettes than types A, B, or AB, which appears to reduce the severity of the disease.

This selective pressure from malaria over thousands of years likely drove up the frequency of type O in regions with historically high malaria burden, particularly sub-Saharan Africa and parts of Central and South America. The practical consequence for forensic work is straightforward: in many populations, O+ is the single most common blood type, which limits how much discriminatory power ABO+Rh typing alone provides. When a suspect is O+, they share that type with a large fraction of the population, reinforcing why extended antigen profiling or DNA analysis is needed for anything approaching individual identification.

Lewis Antigens and the Secretor Connection

The Lewis blood group system interacts closely with both ABO type and secretor status in ways that affect what shows up in body fluids. Lewis antigens are unusual because they are not made by the red blood cell itself. Instead, they are produced by other tissues, released into plasma, and then passively absorbed onto the red cell surface.

The Lewis system is biochemically tied to the same pathways that produce H antigen in secretory tissues. The enzyme responsible for Lewis antigen competes with the secretor enzyme and the ABO enzymes for the same substrate molecule.⁷ The result is that your Lewis type depends partly on whether you are a secretor. Secretors typically express Lewis b antigen, while non-secretors typically express Lewis a. Some people lack both.

For a type O+ suspect who is a secretor, you would generally expect Lewis b antigen on the red cells and in body fluids, alongside soluble H antigen. For a non-secretor, Lewis a would be the expected Lewis type, and H antigen would be absent from secretions. This combination of Lewis type and secretor status gives forensic serologists additional markers that can help narrow down a suspect pool, even within the large group of people who are O+. In practice, DNA profiling has made these distinctions less critical for identification, but Lewis and secretor typing still appear in forensic literature and can be useful when genetic evidence is unavailable or degraded.