Allograft rejection remains one of the central challenges in transplant medicine. Rejection episodes are classified by their timing, underlying immune mechanism, and histological appearance. Understanding these distinctions is essential for selecting appropriate treatment and predicting graft outcomes.
Occurs within minutes to hours
1. Hyperacute Rejection
Hyperacute rejection (HAR) is the most dramatic and historically devastating form of graft rejection. It occurs within minutes to hours of reperfusion and is characterized by immediate graft dysfunction that is not reversible. In the era before reliable cross-matching, hyperacute rejection was a catastrophic outcome that necessitated emergency graft removal.
Mechanism
HAR is mediated by pre-formed, circulating donor-specific antibodies (DSAs) that the recipient already possesses at the time of transplantation. These antibodies arise from prior sensitization events including:
- Prior blood transfusions
- Previous pregnancies (maternal sensitization to fetal HLA antigens)
- Prior transplants
- ABO blood group incompatibility
Upon reperfusion of the graft, pre-formed antibodies immediately bind to donor endothelial antigens (primarily HLA class I antigens and ABO antigens). This triggers rapid complement activation via the classical pathway, producing C3a, C4a, and C5a (anaphylatoxins) and the membrane attack complex (MAC, C5b-9).
The cascade results in:
- Endothelial cell activation and injury
- Platelet aggregation and thrombosis of graft vasculature
- Neutrophil recruitment and degranulation
- Diffuse intravascular coagulation within the graft
- Complete ischemia and infarction of the transplanted organ
Clinical Presentation
In kidney transplantation, the graft becomes swollen, mottled, and cyanotic within minutes of reperfusion — visibly in the operating room. Urine output ceases immediately. The graft is non-salvageable and must be removed.
In heart transplantation, the organ may fail to resume effective contractile function after reperfusion.
Prevention
HAR is effectively prevented by:
- ABO compatibility testing — required for all solid organ transplants
- Complement-dependent cytotoxicity (CDC) crossmatch — detects pre-formed antibodies against donor T and B cells
- Flow cytometry crossmatch (FCXM) — more sensitive, detects lower-level DSAs
- Virtual crossmatch — using single-antigen bead (SAB) testing to identify DSAs against donor HLA specificities
Due to modern crossmatching protocols, true hyperacute rejection is now exceedingly rare in clinical practice.
Occurs days to weeks post-transplant
2. Acute Cellular Rejection (ACR)
Acute cellular rejection is the most common form of rejection in the early post-transplant period, typically occurring within the first days to months after transplantation. Unlike hyperacute rejection, ACR is generally reversible with prompt intensification of immunosuppression.
Mechanism
ACR is primarily mediated by T lymphocytes — both CD4+ helper T cells and CD8+ cytotoxic T cells. The immune response proceeds through two pathways:
- Direct allorecognition: Recipient T cells recognize intact donor MHC molecules presented by donor antigen-presenting cells (passenger leukocytes) that migrate from the graft. This is the dominant pathway early after transplantation.
- Indirect allorecognition: Recipient antigen-presenting cells process and present donor peptides (derived from shed donor MHC molecules) to recipient T cells. This pathway predominates in chronic rejection.
Activated CD4+ T cells release pro-inflammatory cytokines (IL-2, IFN-gamma, TNF-alpha) that recruit and activate macrophages, natural killer cells, and CD8+ cytotoxic T cells. CD8+ cells directly kill donor parenchymal cells via perforin/granzyme and Fas/FasL pathways.
Histopathology
In kidney transplants, ACR shows:
- Tubulitis — lymphocyte infiltration into tubular epithelial cells (Banff t-score)
- Interstitial infiltrate — mononuclear cells in the interstitium (Banff i-score)
- Intimal arteritis — endothelialitis of arterial walls (Banff v-score, indicates severe ACR)
Clinical Features
Clinical signs include rising serum creatinine, decreased urine output, graft tenderness, fever, and hypertension. However, these signs overlap considerably with other causes of graft dysfunction (infection, drug toxicity, obstruction), making biopsy essential for diagnosis.
Treatment
First-line treatment is high-dose corticosteroids (pulse methylprednisolone). Steroid-resistant rejection is treated with anti-thymocyte globulin (ATG). Most ACR episodes (70–90%) respond to treatment when diagnosed early.
Mediated by donor-specific antibodies
3. Acute Antibody-Mediated Rejection (AMR)
Antibody-mediated rejection (AMR) occurs when the recipient generates or possesses donor-specific antibodies (DSAs) targeting HLA antigens or other donor endothelial antigens. AMR can be acute or chronic and is increasingly recognized as a major cause of late graft loss. It carries a worse prognosis than pure ACR.
Mechanism
DSAs bind to donor endothelial cells expressing the target HLA antigens, leading to:
- Complement activation — C4d deposition in peritubular capillaries is a histological hallmark (though C4d-negative AMR is now recognized)
- Natural killer (NK) cell recruitment via antibody-dependent cellular cytotoxicity (ADCC) through Fc-gamma receptors
- Macrophage activation — tissue macrophages interact with antibody-coated endothelial cells
- Endothelial cell apoptosis, intimal thickening, and microvascular injury
Diagnosis (Banff 2022 Criteria)
AMR diagnosis requires all three criteria:
- Histological evidence of acute tissue injury — microvascular inflammation (glomerulitis, peritubular capillaritis), thrombotic microangiopathy, or acute tubular injury
- Evidence of current/recent antibody interaction — C4d staining in peritubular capillaries (linear or diffuse), or microvascular inflammation score ≥2
- Serological evidence of DSAs — positive single-antigen bead (SAB) testing for HLA or non-HLA DSAs (MICA, AT1R antibodies)
Risk Factors for DSA Development
- Non-adherence to immunosuppression
- Prior sensitization (previous transplants, transfusions, pregnancies)
- HLA mismatch degree
- Sub-therapeutic calcineurin inhibitor levels
- Infectious episodes triggering immune activation
Treatment
AMR treatment targets antibody removal and B cell/plasma cell suppression: plasmapheresis, IVIG, rituximab, bortezomib, and complement inhibition (eculizumab). See the Treatments page for detailed protocols.
Months to years, irreversible
4. Chronic Rejection / Chronic Allograft Dysfunction
Chronic rejection is the leading cause of late graft loss, responsible for the gradual deterioration of transplanted organs over months to years. Unlike acute rejection, chronic rejection is largely irreversible once established, as fibrosis replaces functional graft parenchyma.
Pathophysiology
Chronic rejection results from a combination of immune and non-immune injury:
Immune mechanisms:
- Persistent low-grade T cell alloreactivity (indirect pathway predominates)
- Chronic DSA-mediated endothelial injury and microvascular loss
- Macrophage-driven profibrotic cytokine secretion (TGF-beta, PDGF)
Non-immune mechanisms (also contribute to interstitial fibrosis):
- Calcineurin inhibitor nephrotoxicity (for kidney transplants)
- Hypertension and dyslipidemia
- Recurrent infections (CMV, BK virus)
- Ischemia-reperfusion injury from procurement
- Donor organ quality factors
Histopathology
In kidney transplants, chronic rejection manifests as interstitial fibrosis and tubular atrophy (IF/TA). Additional features include:
- Transplant glomerulopathy — double contours of the glomerular basement membrane (GBM), a hallmark of chronic AMR
- Peritubular capillary basement membrane multilayering
- Arteriosclerosis — intimal thickening of interlobular arteries
- Interstitial fibrosis graded by Banff ci-score and ct-score
In heart transplants: cardiac allograft vasculopathy (CAV) — diffuse concentric intimal proliferation affecting the entire coronary tree, distinct from native atherosclerosis.
In lung transplants: bronchiolitis obliterans syndrome (BOS) — progressive airflow obstruction from obliterative fibrosis of small airways.
Prevention Strategies
- Adequate immunosuppression adherence
- DSA monitoring and early treatment
- Minimizing CNI toxicity (consider switch to belatacept or mTOR inhibitors)
- Aggressive cardiovascular risk factor management
- Prophylaxis and treatment of CMV and BK virus
- Protocol biopsies to detect subclinical rejection
The Banff Classification System
The Banff Classification is an internationally standardized schema for grading allograft pathology. First established in Banff, Canada in 1991, the classification is updated regularly through international consensus conferences. It provides pathologists and clinicians with a common language for reporting and comparing biopsy findings.
Kidney Transplant Biopsy — Banff Lesion Scores
Each histological lesion is scored 0–3:
| Lesion | Banff Code | 0 | 1 (Mild) | 2 (Moderate) | 3 (Severe) |
|---|---|---|---|---|---|
| Interstitial infiltrate | i | <10% inflamed cortex | 10–25% | 26–50% | >50% |
| Tubulitis | t | No mononuclear cells in tubules | 1–4 cells/tubule cross-section | 5–10 cells | >10 cells or destruction |
| Intimal arteritis | v | Absent | <25% luminal area | ≥25% luminal area | Transmural arteritis |
| Glomerulitis | g | No glomerulitis | ≤25% glomeruli | 26–75% glomeruli | >75% glomeruli |
| Peritubular capillaritis | ptc | Absent | ≤3 cells/ptc lumen | ≥4 cells, <10% | ≥4 cells, ≥10% ptc |
| Interstitial fibrosis | ci | <6% cortical area | 6–25% | 26–50% | >50% |
| Tubular atrophy | ct | <6% of tubules | 6–25% | 26–50% | >50% |
Banff Diagnostic Categories
| Category | Diagnosis | Key Criteria |
|---|---|---|
| 1 | Normal or non-specific changes | No rejection features |
| 2 | Antibody-mediated changes | C4d+, DSA+, microvascular inflammation |
| 3 | Borderline / suspicious ACR | t1–t2 + i1–i2 but insufficient for ACR |
| 4 | T cell-mediated rejection (TCMR) | i2–3 + t2–3 ± v1–3 |
| 5 | Interstitial fibrosis and tubular atrophy | IF/TA with/without inflammation |
| 6 | Other changes | Not due to rejection (drug toxicity, pyelonephritis, recurrence) |
Note: The Banff schema is continually updated. The 2022 Banff Report introduced significant revisions to AMR criteria, recognizing C4d-negative AMR and refining microvascular inflammation thresholds. Pathologists should use the most current iteration.
Comparative Overview
| Feature | Hyperacute | Acute Cellular | Acute AMR | Chronic |
|---|---|---|---|---|
| Timing | Minutes–hours | Days–months | Days–years | Months–years |
| Mediator | Pre-formed DSAs + complement | T cells (CD4+ / CD8+) | De novo DSAs + complement | T cells + DSAs + non-immune |
| Reversibility | No — graft loss | Yes (70–90% with treatment) | Partial (30–60%) | No (fibrosis permanent) |
| Key histology | Thrombosis, infarction | Tubulitis, interstitial infiltrate | Microvascular inflammation, C4d | IF/TA, transplant glomerulopathy |
| Prevention | Crossmatch, ABO typing | Maintenance IS, monitoring | DSA surveillance, adherence | Optimal IS, risk factor management |
| Treatment | Graft removal | Pulse steroids, ATG | PLEX, IVIG, rituximab | No cure; slow progression |