Organ rejection is the process by which a transplant recipient’s immune system identifies a donated organ as foreign and mounts an attack against it. The response is driven largely by differences in a set of proteins on cell surfaces that act as identity markers, and it can range from a sudden, catastrophic immune assault within minutes of surgery to a slow, silent scarring that unfolds over years. Nearly every transplanted organ faces some degree of this risk, and managing it is the central challenge of transplant medicine.
Why the Immune System Attacks a Transplant
Your immune system is built to distinguish “self” from “not self.” When tissue from another person is placed inside your body, immune cells detect unfamiliar surface proteins on the donor cells. The most important of these are called human leukocyte antigens, or HLA. Every person carries a unique combination of HLA types inherited from their parents, and the greater the mismatch between a donor’s HLA and a recipient’s, the more aggressive the immune response tends to be. The mismatch essentially recruits more immune cells to join the attack, making rejection more severe.1Immunology Letters. Matching and cross-matching The role of HLA matching in transplantation
This is why transplant teams try to match donor and recipient HLA types as closely as possible before surgery. Better matching leads to fewer rejection episodes, longer graft survival, and the possibility of using lower doses of immune-suppressing drugs afterward. Mismatches, on the other hand, mean more rejection episodes, heavier immunosuppression, and a higher risk of the side effects that come with those drugs, including infection and certain cancers.2PubMed Central. HLA Mismatching Strategies for Solid Organ Transplantation – A Balancing Act
HLA genes are not the whole story, though. Non-HLA genetic differences also contribute, and even well-matched transplants can face rejection. The immune response involves more than just the adaptive arm of the immune system, meaning the T cells and antibodies that learn to recognize specific threats. Innate immune cells, the body’s first responders that react more broadly to anything unusual, also play a role. Some of these innate cells can even distinguish donor tissue from the recipient’s own cells, a capability researchers once thought belonged exclusively to adaptive immune cells.3PubMed Central. Innate immune cells in transplantation
Types of Organ Rejection
Rejection is not a single event. It is classified by timing and mechanism into three broad categories, each with different causes, urgency, and outlook.
Hyperacute Rejection
This is the rarest and most dramatic form. It occurs within minutes to hours after the transplanted organ is connected to the recipient’s blood supply. Hyperacute rejection happens when the recipient already has antibodies circulating in their blood that recognize and immediately attack the donor tissue. These pre-formed antibodies bind to the blood vessel lining of the new organ and activate a cascade of inflammatory proteins called complement, which rapidly destroys the organ’s vasculature. Research on cross-species transplants has confirmed that it is the antibodies, not complement acting alone, that initiate this process: removing the offending antibodies before surgery significantly prolonged graft survival even when complement levels remained normal.4PubMed Central. Mechanism of complement activation in the hyperacute rejection of porcine organs transplanted into primate recipients
Modern crossmatch testing, which checks the recipient’s blood for antibodies that react against the donor’s cells, has made hyperacute rejection extremely uncommon in human-to-human transplants. When it does occur, the organ is usually lost and must be removed.
Acute Rejection
Acute rejection is far more common and typically arises within weeks to months after transplantation, though it can happen at any time. It comes in two main flavors. The first is cellular rejection, driven primarily by T cells that infiltrate the graft and directly attack its cells.5PubMed Central. Mechanism of cellular rejection in transplantation The second is antibody-mediated rejection, in which the recipient produces new antibodies against donor HLA or other targets on the graft, triggering damage to the organ’s small blood vessels.
Acute cellular rejection is one of the most thoroughly studied problems in transplant medicine. T cells are central, but the response is more complex than a single cell type launching a coordinated strike. Significant diversity exists among the T cell populations involved, and repeated episodes of acute cellular rejection have been linked to the development of chronic graft dysfunction over time.6PubMed Central. An Immune Atlas of T Cells in Transplant Rejection: Pathways and Therapeutic Opportunities
Antibody-mediated rejection can be harder to pin down. In lung transplants, for example, researchers found that roughly six in ten cases of antibody-mediated rejection tested negative for a classic tissue marker (C4d) that clinicians traditionally looked for. Despite the absence of this marker, the clinical presentation and outcomes were similar to cases where the marker was present, making diagnosis tricky if clinicians rely on one test alone.7PubMed Central. The role of C4d deposition in the diagnosis of antibody-mediated rejection after lung transplantation
Chronic Rejection
Chronic rejection is the slow burn. It develops over months to years and is characterized by a gradual thickening of blood vessel walls inside the graft and progressive scarring of the organ’s working tissue. These changes starve the organ of blood flow, leading to shrinkage and eventual failure.8PubMed Central. Chronic rejection. A general overview of histopathology and pathophysiology with emphasis on liver, heart and intestinal allografts Chronic rejection remains the leading cause of late graft loss for many organ types, and current treatments are far less effective against it than against acute episodes.
Symptoms and Warning Signs
One of the frustrating realities of organ rejection is that early symptoms are often vague and easy to dismiss. They overlap heavily with other post-transplant complications like infection, drug side effects, or simple recovery from major surgery. Some general warning signs include fever, fatigue, decreased urine output (for kidneys), shortness of breath (for lungs and hearts), and tenderness or swelling over the transplant site. Jaundice and abnormal liver tests can signal trouble in a liver graft.
The problem is that many of these indicators lack the precision clinicians need. Standard liver blood tests, for example, have low sensitivity and specificity for graft rejection and correlate poorly with how severe the rejection actually is on a tissue level.9PubMed Central. Markers of acute rejection and graft acceptance in liver transplantation For kidneys, researchers have found that certain blood markers, like soluble TNF receptor levels, rise earlier than the traditional standby of serum creatinine, potentially flagging rejection before standard tests catch it.10Nephrology Dialysis Transplantation. Value of serum soluble tumour necrosis factor concentrations in the diagnosis and prognosis of renal graft rejection But in practice, biopsy remains the definitive diagnostic tool for most organ types.
In lung transplants, chronic rejection often manifests as a condition called chronic lung allograft dysfunction. The most common form involves a persistent decline in airflow, known as bronchiolitis obliterans syndrome, where scar tissue narrows and blocks the small airways. A less common but more aggressive form involves restrictive changes, where the lung loses its ability to expand fully, and this carries a worse prognosis.11PubMed Central. Detection, classification, and management of rejection after lung transplantation
How Rejection Is Diagnosed
Biopsy, taking a small sample of tissue from the transplanted organ and examining it under a microscope, has been the gold standard for decades. Since the early 1990s, pathologists have used a standardized grading system known as the Banff Classification to evaluate kidney biopsies, looking at specific patterns of inflammation and tissue damage to determine whether rejection is occurring, what type it is, and how severe it has become. That system has been updated regularly and is now used worldwide.12PubMed Central. A 2018 Reference Guide to the Banff Classification of Renal Allograft Pathology Recent updates to the Banff system have incorporated molecular diagnostics, using gene-expression analysis on biopsy tissue to improve accuracy, though the field is still working out exactly which molecular classifiers and thresholds to standardize.13PubMed. The Banff 2022 Kidney Meeting Report: Reappraisal of microvascular inflammation and the role of biopsy-based transcript diagnostics
Biopsies work well, but they are invasive and carry their own risks, including bleeding and infection. They also only capture a snapshot of a small piece of tissue at one moment, so clinicians have long wanted a reliable blood test. One of the most promising recent developments is donor-derived cell-free DNA, or dd-cfDNA. When transplanted cells are injured or dying, they release fragments of their DNA into the recipient’s bloodstream. A blood draw can measure how much of that donor DNA is circulating. Higher levels suggest the graft is under attack. In kidney transplant recipients, adding dd-cfDNA measurement to standard monitoring improved the ability to detect rejection, and it showed strong value for catching subclinical rejection in patients who appeared stable by other measures.14Nature Medicine. Cell-free DNA for the detection of kidney allograft rejection Studies in pediatric kidney transplant recipients have confirmed these findings, showing that dd-cfDNA levels rose with more severe inflammation and independently predicted the presence of rejection.15PubMed. Donor-derived Cell-free DNA as a Noninvasive Biomarker of Kidney Allograft Rejection in Pediatric Kidney Transplantation
Treating Rejection Episodes
When rejection is caught, the treatment depends on which type it is. For acute cellular rejection, the first line of treatment is typically a pulse of high-dose corticosteroids. This brings the immune attack under control in most cases. When steroids are not enough, more powerful agents that deplete T cells can be used.16PubMed Central. Current Therapies in Kidney Transplant Rejection
Antibody-mediated rejection is trickier to treat. Plasmapheresis, which filters the offending antibodies out of the blood, is the most common approach, though how well it works remains debated. Other treatments include intravenous immunoglobulins and drugs that target the B cells producing the antibodies, but effectiveness varies and none has a rock-solid evidence base yet.16PubMed Central. Current Therapies in Kidney Transplant Rejection
A large systematic review of rejection treatment across various types of transplants found that corticosteroids were used in roughly seven out of ten rejection episodes, confirming their central role. Tacrolimus, another immunosuppressive drug, was the second most common choice. Most episodes required at least two drugs used together, most frequently a corticosteroid combined with topical tacrolimus.17PubMed Central. A systematic review of treatment strategies to combat acute and chronic rejection episodes in vascularized composite allotransplantation
The Infection Connection
A counterintuitive twist in transplant medicine is that infections can actually fuel rejection. Cytomegalovirus, or CMV, is a common virus that most healthy people carry without ever knowing it, but in immunosuppressed transplant recipients it can reactivate and cause serious illness. Beyond the direct damage CMV causes, it also appears to stir up the immune system in ways that raise the risk of acute rejection. A meta-analysis pooling data from dozens of studies found that transplant recipients infected with CMV had about twice the odds of developing acute rejection compared to those without the virus. The effect was seen across liver, kidney, and lung transplants, and was strongest in liver recipients, where the odds were more than four times higher.18PubMed Central. Cytomegalovirus infection contributes to acute rejection in solid organ transplant recipients: a systematic review and meta-analysis
Smaller studies reinforce the pattern. In one cohort of kidney transplant recipients, about a third of patients with CMV disease experienced graft rejection, compared to fewer than one in ten in a control group without CMV.19PubMed Central. The effect of cytomegalovirus infection on acute rejection in kidney transplanted patients This creates a difficult balancing act for clinicians: the same immunosuppressive drugs that prevent rejection also leave patients vulnerable to infections like CMV, which can then promote rejection in return.
Why Taking Medications Matters
After a transplant, you will likely take immunosuppressive medications for the rest of your life, or for as long as the organ functions. These drugs are what keep your immune system from mounting a full-scale attack against the graft. Skipping doses, stopping early, or taking medications inconsistently is one of the most preventable causes of late graft loss, and it happens more than transplant teams would like.
Non-adherence to immunosuppressive medications is influenced by a tangle of factors. Demographic, psychological, and transplant-specific variables all play a role.20PubMed. Non-adherence to immunosuppressive medications in kidney transplant recipients- a systematic scoping review Research has identified certain risk factors that consistently predict poor adherence, including a history of psychiatric disorders, prior non-adherence to other medical regimens, and lack of supplemental private insurance.21PubMed Central. Kidney Transplantation Assessing Risk Factors and Posttransplant Outcomes of Nonadherence Among Kidney Transplant Recipients Side effects from the drugs themselves, including weight gain, tremor, diabetes, and increased susceptibility to infection, can make long-term adherence even harder. Transplant programs increasingly screen for these risk factors before surgery and build support systems around patients who are at higher risk of drifting off their medication schedule.
The Quest for Tolerance
The ultimate goal in transplant medicine is not just to manage rejection but to eliminate the need for lifelong immunosuppression altogether. Researchers call this “transplant tolerance,” a state where the recipient’s immune system accepts the graft as its own without ongoing drug therapy. It sounds aspirational, but it has been documented in real patients. Some kidney and liver recipients whose immunosuppressive drugs were stopped, whether intentionally for medical reasons or because the patient simply stopped taking them, have maintained perfectly normal graft function for years without any medication.22PubMed Central. Transplantation tolerance
This state, sometimes called clinical operational tolerance, remains rare and unpredictable. It is characterized by normal graft function in the complete absence of immunosuppressive drugs, but there is currently no reliable way to identify in advance which patients will achieve it.23PubMed. Clinical operational tolerance after solid organ transplantation The liver appears to be the organ most likely to reach this state naturally, possibly because of unique immunological properties of the liver itself. Kidney tolerance has been achieved in experimental protocols involving bone marrow transplants from the same donor, but these approaches are complex, carry their own risks, and remain far from routine clinical use.
Xenotransplantation and Genetically Modified Donor Organs
The severe shortage of human donor organs has pushed researchers to explore an unconventional solution: using organs from other species, particularly pigs. The field is called xenotransplantation, and the rejection barrier between species is even higher than between two unrelated humans. Most current efforts focus on genetically modifying donor pigs to make their organs less visible to the human immune system.24PubMed Central. Genetically engineered pigs for xenotransplantation: Hopes and challenges
Progress has been substantial. Researchers have developed pigs carrying ten genetic modifications: four pig genes knocked out and six human genes inserted. The pig gene deletions remove surface molecules that would otherwise trigger an immediate human immune response, while the human gene insertions help the pig organ’s cells communicate “do not attack” signals to the recipient’s immune system. Hearts and kidneys from these pigs have been studied in preclinical models with brain-dead human recipients, and prolonged survival of pig organs in non-human primates has been achieved in laboratory settings.25PubMed Central. Physiological basis for xenotransplantation from genetically modified pigs to humans A small number of compassionate-use pig kidney and heart transplants into living human patients have taken place, generating enormous public interest and yielding the first real-world data on how these organs perform in human bodies. The rejection challenges are formidable but increasingly tractable, and xenotransplantation represents one of the most active frontiers in the field.