Who Can Donate a Kidney to an O+ Blood Type Recipient?

Under standard compatibility rules, only donors with blood type O can donate a kidney to an O+ recipient. That restriction exists because people with type O blood carry antibodies against both A and B antigens, meaning a kidney from a type A, B, or AB donor would trigger an immune attack. But the full picture is more flexible than that simple rule suggests. Paired exchange programs, desensitization protocols, and even a little-known blood subtype exception have expanded the donor pool for O+ recipients in ways that many people never hear about during early conversations with transplant teams.

Why Type O Recipients Are Restricted

Your blood type is determined by sugar molecules (antigens) sitting on the surface of your cells. Type A blood has A antigens, type B has B antigens, AB has both, and O has neither. Your immune system treats any antigen it doesn’t recognize as a foreign invader. Because O blood has neither A nor B antigens, people with type O develop antibodies against both. If you transplanted a type A or B kidney into an O recipient without any special preparation, those antibodies would immediately attack the donor organ’s blood vessels.

This is why, under conventional matching, an O+ recipient needs an O donor. The reverse is not true: a type O kidney is safe for recipients of any blood type, since it carries no A or B antigens to provoke an immune response. That mismatch in supply and demand creates a structural disadvantage. O kidneys from deceased donors sometimes get allocated to non-O recipients, and one study found that about 14% of deceased-donor O kidneys went to non-O recipients, contributing to a growing backlog of O patients on the waiting list.¹ Type O recipients already face the longest wait times for a deceased-donor kidney, a problem that transplant policy researchers have flagged as a fairness concern.²

Does the Rh Factor Matter?

If you’re O+, you might wonder whether only Rh-positive donors are eligible. Here the answer is refreshingly simple: the Rh factor (the “+” or “−” after your blood type) is not a barrier in kidney transplantation. Unlike blood transfusions, where Rh mismatches can cause serious reactions, kidney tissue does not provoke the same Rh-driven immune response. An O− donor can give a kidney to an O+ recipient, and vice versa, without added risk of rejection. One early study even found that Rh-positive recipients tended to have slightly better graft outcomes, but this has not led to any clinical restriction on Rh-mismatched donations.³ In practice, transplant centers do not screen for Rh compatibility when evaluating kidney donors. So for an O+ recipient, any O donor, positive or negative, is a standard-compatible match.

The A2 Subtype Exception

There is one situation where a donor who is technically “type A” can safely donate to an O recipient, and it involves a less well-known blood subtype. Most people with type A blood have the A1 subtype, which produces a dense layer of A antigens on cell surfaces. But a smaller fraction have the A2 subtype, which produces far fewer A antigens. Among type A donors registered with one national kidney registry, about 13% were confirmed to be A2 after genetic testing.⁴

Because A2 kidneys carry so few A antigens, some O recipients can tolerate them without a severe immune reaction, provided their anti-A antibody levels are low enough. One transplant center began preferentially allocating A2 kidneys to B and O recipients starting in 1991, selecting patients whose anti-A antibody titers were at or below a low threshold.⁵ This practice has expanded at many programs since then. For O+ recipients with low anti-A antibodies, an A2 donor represents a real option that the basic “O-to-O only” rule overlooks. The challenge is that A2 subtyping is not always performed routinely, so some compatible donors get missed entirely.⁴

ABO-Incompatible Transplantation Through Desensitization

What if you have a willing living donor who is type A1, B, or AB? Until the early 2000s, this combination was essentially a dead end. Today, ABO-incompatible kidney transplantation is a real option at experienced transplant centers. The strategy involves preparing the recipient’s immune system ahead of surgery to remove or suppress the antibodies that would otherwise attack the mismatched kidney.

A typical desensitization protocol combines several approaches: rituximab (a drug that depletes the immune cells responsible for making antibodies), plasmapheresis (filtering antibodies out of the blood), and sometimes intravenous immunoglobulin. The specifics vary by center, but some programs tailor the intensity to the recipient’s antibody levels. Patients with low antibody titers against the donor’s blood type may need only oral medications, while those with high titers may require the full combination of rituximab, plasmapheresis, and immunosuppressants.⁶ After transplantation, the recipient’s antibody levels are monitored; if they rebound above a certain threshold, additional plasmapheresis sessions may be needed.⁶

The results have been encouraging. Recent reports show that outcomes after ABO-incompatible kidney transplants are now similar to those of standard-compatible transplants.⁷ That said, the process is more involved, more expensive, and requires a transplant team with specific expertise. Not every center offers it, and not every recipient is a good candidate. But for an O+ patient whose only willing donor has an incompatible blood type, desensitization has changed the conversation from “impossible” to “let’s evaluate.”

Paired Kidney Exchange Programs

Desensitization is one way to overcome a blood type mismatch. The other major route is to sidestep the mismatch entirely through a swap. In a paired kidney exchange, two or more donor-recipient pairs who are individually incompatible trade donors so that each recipient gets a compatible kidney. Imagine you’re O+ and your willing donor is type A. Somewhere else, an A recipient has an O donor they can’t use because of a different incompatibility (say, a positive crossmatch). You receive the O kidney, and the A recipient gets the A kidney. Both transplants proceed with standard blood-type compatibility.

The simplest version of this is a two-way swap between pairs with opposite ABO mismatches. But exchanges can also be “unbalanced,” where a pair that is compatible but not identical (for example, an O donor paired with an A recipient) agrees to swap with a truly incompatible pair (an A donor and O recipient), giving the O recipient access to a compatible organ.⁸ More complex chains can involve three, four, or even dozens of pairs, and “unconventional” paired donations allow O donors and O recipients to participate in exchanges by matching across positive crossmatches rather than just blood type barriers.⁹

Paired exchange does face logistical challenges. The larger the registry of available pairs, the better the odds of finding a match. Since the concept gained traction in the 1980s, programs have worked through geographic barriers, legal hurdles, and the tricky question of whether requiring simultaneous surgeries is practical when paired donors are at hospitals across the country.¹⁰ Despite these complexities, paired exchange has become one of the most effective tools for getting O+ recipients to transplant when their direct donor doesn’t match.

Altruistic Donor Chains

Paired exchanges get even more powerful when they start with a nondirected (altruistic) donor: someone who volunteers a kidney without a specific recipient in mind. This person donates to a stranger, whose incompatible donor then gives to another stranger, whose incompatible donor gives to the next, and so on down the line. These are called donor chains, and they can stretch remarkably long.

Altruistic donors with blood type O are especially valuable because their kidneys are universally compatible, which means the chain can extend further before hitting a dead end. In one analysis, 40 blood type O nondirected donors triggered chains that averaged six transplants each, with the longest chain reaching 30 transplants.¹¹ That same analysis found that 133 blood type O patients received kidneys through chains initiated by just 77 nondirected donors.¹¹

There has been some concern that chains might actually hurt O patients on the deceased-donor waiting list, because an O kidney is so useful for extending a chain that it rarely gets “donated off” to the waiting list at the end. But research has found the opposite: starting chains with O nondirected donors actually results in more O patients receiving transplants than if those donors had simply given directly to someone on the waiting list.¹² For O+ recipients, the expanding network of paired exchange registries and donor chains has been a meaningful step toward closing the supply gap.

Beyond Blood Type: Other Compatibility Factors

Blood type compatibility is the first hurdle, but it’s not the only one. Even if a potential donor has the right blood type, two additional tests determine whether the transplant can proceed safely.

The first involves human leukocyte antigens (HLA), proteins on the surface of cells that your immune system uses to distinguish “self” from “foreign.” Everyone inherits a unique combination of HLA markers. When a donor kidney has very different HLA markers from the recipient, the recipient’s immune system is more likely to mount a rejection response. HLA antibodies in the recipient’s blood are significant risk factors for both acute rejection and long-term graft loss.¹³ Modern immunosuppressive drugs have reduced the impact of HLA mismatches compared with decades past, but a closer match still tends to produce better long-term outcomes.

The second is the crossmatch test, which mixes the recipient’s blood serum with the donor’s cells to look for a direct immune reaction. If the recipient has preformed antibodies that bind to the donor’s cells, the crossmatch is “positive,” and the transplant generally cannot proceed without additional treatment. A positive crossmatch means the recipient’s immune system has already been primed (through prior transplants, blood transfusions, or pregnancy) to attack that particular donor’s tissue.¹⁴ The preliminary evaluation of any living donor includes blood type verification, HLA typing, and crossmatching against the intended recipient.¹⁵

Living Donors Versus Deceased Donors for O+ Recipients

Because O+ recipients face longer waits for a deceased-donor kidney, finding a living donor can be transformative, and not just because it shortens the wait. Kidneys from living donors consistently outperform deceased-donor kidneys. A large UK national cohort study found that living-donor recipients had a roughly 6% lower risk of graft failure at five years compared with deceased-donor recipients (about 5.6% versus 11.6%), and the benefit grew over time.¹⁶ That advantage held even when the living donor was older or when HLA matching was less than ideal.¹⁶ Another study found significantly shorter overall patient and graft survival in deceased-donor recipients after adjusting for factors like age and diabetes.¹⁷

The reasons are straightforward. A kidney from a living donor typically spends less time without blood flow, is screened more thoroughly, and comes from someone who has been evaluated as healthy enough to donate. For O+ patients who might otherwise spend years on dialysis waiting for a deceased-donor organ, pursuing a living donor through direct donation, paired exchange, or a donor chain offers both shorter waits and better expected outcomes.

What Donors Should Know About Long-Term Safety

People considering kidney donation for an O+ loved one understandably worry about what happens to them after surgery. The evidence here is somewhat mixed, and honesty about that matters. One large study followed donors for an average of about 12 years and found that their survival was similar to matched controls in the general population. Most donors maintained healthy kidney function, and their quality-of-life scores were actually better than population norms.¹⁸ The rate of end-stage kidney disease among donors in that study was lower than in the general population.

However, a different long-term study comparing donors with carefully matched non-donors who would have been eligible for donation painted a less rosy picture. It found a significantly increased risk of end-stage kidney disease, cardiovascular death, and all-cause death among donors compared with these highly matched controls.¹⁹ The overall incidence of kidney failure among donors remained low in absolute terms, but the relative risk was meaningfully elevated. The difference between these two studies largely comes down to the comparison group: donors look fine compared with the general population (which includes many unhealthy people), but look slightly worse compared with similarly healthy people who kept both kidneys.

Potential donors should also be aware of the practical costs. A multi-center study found that the vast majority of living kidney donors incur direct out-of-pocket expenses, and over a third reported lost wages in the first year after donation. Among those who lost wages, the median amount exceeded $2,700. Perhaps more troubling, about a third of donors reported that a family member or friend had been willing to consider donating but backed out over concerns about lost income, and the majority of those individuals never even completed the evaluation process.²⁰ Financial barriers are a real, if underappreciated, obstacle to expanding the living donor pool for O+ recipients.

Allocation Policy and the O Disadvantage

The structural disadvantage for O recipients isn’t just a quirk of biology. It’s embedded in how deceased-donor organs are allocated. Because O kidneys are compatible with everyone, they can be directed to any patient on the list. But A, B, and AB kidneys cannot flow back to O recipients, creating a one-way street that drains the O supply while limiting replenishment. Allocation systems in various countries have tried to address this imbalance. A Korean nationwide study found that O recipients and highly sensitized patients had similar post-transplant outcomes to other groups, despite their reduced access to deceased-donor kidneys. The authors used this finding to argue for a new scoring system that would give extra priority points to these disadvantaged groups.²¹

In the United States, the United Network for Organ Sharing has also adjusted its allocation formulas over the years to reduce the blood type O penalty, including rules that generally restrict O deceased-donor kidneys to O recipients except in specific circumstances. These policy adjustments help, but they haven’t eliminated the gap entirely. For O+ patients, the practical message remains the same: pursuing living donation, paired exchange, or enrollment in a chain program remains the most proactive path toward transplant.

Enzymatic Conversion of Donor Kidneys

One of the most intriguing areas of transplant research could eventually make the entire blood type question less important. Scientists have demonstrated in laboratory settings that they can strip A or B antigens from donor kidneys using enzymes, effectively converting them into type O organs. In one study, researchers used a pair of bacterial enzymes to remove blood group A antigens from the vasculature of human kidneys during machine perfusion outside the body. Simulated transplants afterward showed no significant antibody-mediated injury.²² A separate team applied a similar approach to type B kidneys, using a different enzyme during cold perfusion, and also prevented the hyperacute rejection that normally occurs in ABO-incompatible transplants.²³

If this technology reaches clinical use, it could theoretically turn any donated kidney into a universal organ. That would dissolve the blood type matching problem at its root and dramatically shorten wait times for O recipients. The research is still in its early stages, with work so far limited to ex vivo models and animal simulations. But the concept is far enough along to generate real optimism among transplant researchers.

Pig Kidney Xenotransplantation

Beyond enzymatic conversion, another frontier aims to expand the organ supply altogether rather than just reshuffle it. Genetically modified pig kidneys have been transplanted into human recipients in a handful of closely watched cases over the past few years. Researchers have specifically flagged blood type O patients as among those who stand to gain the most from this technology, because they face the longest waits and the steepest deceased-donor shortages. One recent proposal suggested that the first formal clinical trials of pig kidney transplantation should target patients aged 55 to 65 with blood type O or B who are unlikely ever to receive a human deceased-donor kidney.²⁴ Xenotransplantation remains experimental and faces enormous scientific and regulatory hurdles, but the fact that O+ patients are explicitly named as priority candidates speaks to how severe the supply problem is for this group and how urgently the field is looking for ways around it.

1
PubMed. The ‘blood group O problem’ in kidney transplantation–time to change?
2
Transplantation. Restricting Living-Donor–Cadaver-Donor Exchanges to Ensure that Standard Blood Type O Wait-List Candidates Benefit
3
Tissue Antigens. Some Important Factors in Cadaver‐Donor Kidney Transplantation
4
PubMed Central. Discrepant subtyping of blood type A2 living kidney donors: Missed opportunities in kidney transplantation
5
PubMed. Ten-year experience in transplantation of A2 kidneys into B and O recipients
6
Clinical and Experimental Transplantation. Laboratory approaches in desensitization for ABO-incompatible transplantation
7
PubMed Central. Current protocols and outcomes of ABO-incompatible kidney transplantation
8
PubMed Central. Living donor kidney paired exchange: An observational study
9
JAMA. Clinical Results From Transplanting Incompatible Live Kidney Donor/Recipient Pairs Using Kidney Paired Donation
10
PubMed. Kidney paired donation: fundamentals, limitations, and expansions
11
JAMA Surgery. Kidney Transplant Chains Amplify Benefit of Nondirected Donors
12
Nature Reviews Nephrology. Use of altruistic nondirected donors in kidney transplant chains maximizes transplant numbers
13
PubMed Central. Utility of HLA Antibody Testing in Kidney Transplantation
14
PubMed. The lymphocyte crossmatch by flow cytometry for kidney transplantation
15
American Journal of Kidney Diseases. Evaluation of the Living Kidney Donor: Guidelines Straight to the Point
16
PubMed Central. Comparison of outcomes after living and deceased donor kidney transplantation: UK national cohort study
17
PubMed Central. Does Kidney Transplantation With Deceased or Living Donor Affect Graft Survival?
18
PubMed Central. Long-term consequences of kidney donation
19
PubMed. Long-term risks for kidney donors
20
PubMed Central. The Living Donor Lost Wages Trial: Study Rationale and Protocol
21
PubMed Central. A novel allocation scheme for deceased donor kidneys to balance equity and utility
22
Nature Communications. Enzymatic conversion of human blood group A kidneys to universal blood group O
23
PubMed Central. Enzymatic conversion of blood group B kidney prevents hyperacute antibody-mediated injuries in ABO-incompatible transplantation
24
Kidney Medicine. Progress in Pig Kidney Xenotransplantation and Prospects for Clinical Trials