Transplant Immunology and Rejection

Antibody-mediated (pre-formed or de novo) mechanisms

• Prior allosensitization: exposure to foreign HLA through pregnancy, blood transfusion, or a previous transplant generates memory B cells and circulating anti-HLA IgG — the substrate for hyperacute and early antibody-mediated rejection.
• ABO incompatibility: naturally occurring isohemagglutinins (IgM) bind donor endothelial ABO antigens and fix complement immediately; this is why ABO typing and crossmatch are non-negotiable.
• De novo donor-specific antibody: usually emerges in the setting of under-immunosuppression, and is the dominant driver of late AMR and chronic vasculopathy.

T-cell (cellular) mechanisms

• HLA mismatch burden: greater numbers of mismatched A, B, and DR loci increase direct and indirect allorecognition and correlate with both acute rejection and long-term graft loss.
• Inflammatory graft injury as an adjuvant: prolonged cold ischemia time, donation after circulatory death, and delayed graft function upregulate MHC and costimulatory molecules, lowering the threshold for T-cell priming.

Non-modifiable risk factors

• Degree of HLA mismatch, retransplantation, and a high calculated PRA.
• Younger recipient age (more robust alloimmunity) and recipient ancestry — CYP3A5 expressor genotypes, more frequent in patients of African ancestry, accelerate tacrolimus metabolism and yield subtherapeutic troughs.
• Deceased-donor rather than living-donor graft.

Modifiable risk factors (where examiners hide the answer)

• Non-adherence to immunosuppression: the single most important modifiable driver, classically an adolescent or young adult in the second post-transplant year with a missed clinic visit and an undetectable tacrolimus trough.
• Subtherapeutic drug levels from interactions: rifampin, carbamazepine, and St. John's wort induce CYP3A4 and drop calcineurin inhibitor levels; conversely azoles, diltiazem, and macrolides cause toxicity.
• Infection-driven immune activation: CMV and BK viremia (and the immunosuppression reduction they prompt) precipitate rejection.
• Steroid or CNI withdrawal protocols and prior treated acute rejection episodes, which predispose to chronic allograft dysfunction.

Hyperacute Rejection (minutes to hours)

• Pre-existing IgG or IgM antibodies against donor HLA antigens (or ABO blood group antigens in ABO-incompatible transplants) bind to donor endothelium
• Complement activation via classical pathway (C1q binding) leads to C3 and C5 deposition
• Membrane attack complex (MAC/C5b-9) formation causes immediate endothelial injury and thrombosis
• Results in irreversible graft necrosis within minutes to hours; graft loss is virtually guaranteed
• Prevented entirely by pre-transplant crossmatching (serum incubation with donor lymphocytes)

Acute Rejection (days to months)

• Acute cellular rejection (ACR): T-cell mediated, most common type of acute rejection
• Recipient CD8+ cytotoxic T lymphocytes recognize donor MHC Class I antigens directly on donor antigen-presenting cells (APCs) via allorecognition
• Recipient CD4+ helper T cells recognize donor MHC Class II antigens
• IL-2 production drives T-cell proliferation and expansion
• Infiltrating T cells cause direct cytotoxic injury to graft parenchyma and vasculature
• Macrophages and NK cells also contribute to tissue damage

• Acute antibody-mediated rejection (AMR): Humoral immune response
• De novo development of donor-specific antibodies (DSAs) against HLA or non-HLA antigens
• DSA-complement complexes deposit in graft vasculature (especially glomeruli in kidney grafts)
• Endothelial activation and injury with C4d deposition (marker of AMR)
• Thrombotic microangiopathy and vasculitis

Chronic Rejection (months to years)

• Mixed cellular and humoral immune responses persist at lower level
• Chronic allograft nephropathy (CAN) in kidney: progressive interstitial fibrosis and tubular atrophy (IF/TA)
• Chronic vasculopathy/intimal proliferation: smooth muscle cell proliferation in graft vessels
• Obliterative bronchiolitis in lung transplants: airway fibrosis and obliteration
• TGF-β, PDGF, and other growth factors mediate fibroproliferative response
• Often called "graft versus host disease-like" response due to persistent immune activation

• Donor immune cells (especially T cells) recognize recipient tissue as foreign
• More common with increased HLA mismatch and T-cell replete grafts
• Acute GVHD: skin, gut, liver involvement within first 100 days
• Chronic GVHD: can develop after day 100, fibrotic complications

Hyperacute Rejection

• Immediate graft failure in operating room or immediately post-operative (within minutes to hours)
• Graft becomes cyanotic, mottled, and flaccid
• Rapid rise in serum creatinine (kidney transplant) or organ dysfunction markers
• Thrombosis evident on graft biopsy or gross inspection

Acute Cellular Rejection

• Fever (often low-grade)
• Elevated allograft enzymes: AST/ALT (liver), lipase (pancreas), troponin (heart)
• Organ-specific symptoms: oliguria and rising creatinine (kidney); dyspnea and decreased ejection fraction (heart); abdominal pain and elevated liver enzymes (liver)
• Delayed presentation: typically 5 days to 3 months post-transplant (though can occur any time if immunosuppression inadequate)
• May be clinically silent early; requires high index of suspicion

Acute Antibody-Mediated Rejection

• Similar organ-specific symptoms as ACR but may have more rapid deterioration
• Proteinuria and hematuria prominent (kidney transplant)
• Can occur very early (within days) if pre-sensitized
• More resistant to standard corticosteroid therapy than ACR

Chronic Rejection

• Insidious, progressive graft dysfunction over months to years
• Gradual creatinine rise (kidney), declining ejection fraction (heart), declining FEV1 (lung)
• Often asymptomatic until advanced
• Progressive proteinuria (kidney)
• May be associated with recurrent, inadequately treated acute rejection episodes

Important Clinical Pearls About Presentation

• Acute rejection may present with minimal or non-specific symptoms; maintain high clinical suspicion with any unexplained change in graft function
• Time to presentation helps narrow differential: hyperacute (minutes-hours), acute (days-months), chronic (months-years)
• Patient compliance with immunosuppression is the strongest modifiable risk factor for rejection

Serologic Testing

• Donor-Specific Antibodies (DSAs): Detection of IgG or IgM antibodies against donor HLA Class I (A, B) or Class II (DR, DQ) antigens using solid-phase assays (Luminex platform most common); presence indicates allosensitization and increases rejection risk
• Complement-binding DSAs: More clinically significant than non-complement-binding DSAs in predicting AMR
• Panel Reactive Antibodies (PRA): Percentage of donor population against which recipient's serum reacts; high PRA indicates significant allosensitization
• Pre-transplant crossmatch: Direct testing of recipient serum against donor lymphocytes; positive crossmatch is absolute contraindication (prevents hyperacute rejection)

• Kidney biopsy (percutaneous or open): Shows interstitial inflammation (ACR), glomerulonephritis (recurrent disease or AMR), IF/TA (chronic rejection)
• Heart biopsy (endomyocardial): Lymphocytic infiltration and myocyte necrosis; performed routinely post-transplant in many centers
• Liver biopsy: Portal tract inflammation, interface hepatitis
• Lung biopsy (transbronchial): Airway and vascular inflammation; obliterative bronchiolitis in chronic rejection
• Banff Classification (international standard): Grades severity of rejection on histology

C4d Staining

• C4d deposition on peritubular capillaries (kidney) or vessel walls indicates complement activation and is hallmark of antibody-mediated rejection
• Present in ~90% of AMR cases
• Helps distinguish AMR from pure cellular rejection

Graft-Specific Biomarkers

• Donor-derived cell-free DNA (dd-cfDNA): Circulating DNA fragments from donor cells; elevated levels suggest allograft injury and correlate with rejection
• Emerging clinical utility for non-invasive rejection monitoring

Functional Assessment

• Serial creatinine and BUN (kidney)
• Echocardiography, troponin, BNP (heart)
• Liver function tests (liver)
• FEV1 trends (lung)
• Any unexplained deterioration warrants biopsy consideration

Important Diagnostic Considerations

• Biopsy is essential to differentiate rejection from other causes of graft dysfunction (

Hyperacute rejection — surgical, not medical

• Immediate graft explantation is the only management; thrombosed necrotic tissue is a source of sepsis and DIC. No immunosuppressive regimen salvages it. A positive crossmatch is a contraindication to proceeding with transplant.

Induction immunosuppression (at implantation)

• IL-2 receptor (CD25) antagonist: basiliximab, for standard immunologic risk.
• Lymphocyte-depleting polyclonal antibody: antithymocyte globulin, favored by KDIGO for high-risk recipients (high PRA, DSA, retransplant, deceased-donor graft).

Maintenance therapy — triple regimen (KDIGO)

• Calcineurin inhibitor: tacrolimus (preferred over cyclosporine), binds FKBP-12 → inhibits calcineurin → blocks NFAT dephosphorylation → no IL-2 transcription.
• Antimetabolite: mycophenolate mofetil, inhibiting inosine monophosphate dehydrogenase, on which lymphocytes uniquely depend.
• Corticosteroid: prednisone, often tapered but rarely withdrawn in immunologically high-risk patients.
• Alternatives: mTOR inhibitor (sirolimus) or the costimulation blocker belatacept (CTLA-4–Ig, blocks CD80/86); belatacept is contraindicated in EBV-seronegative recipients because of PTLD risk.

Acute cellular rejection

• Pulse IV corticosteroid (methylprednisolone) is first-line, then optimize baseline maintenance and confirm therapeutic drug levels.
• Steroid-refractory disease: escalate to antithymocyte globulin.

• Antibody removal and modulation: therapeutic plasma exchange plus IVIG, commonly with the anti-CD20 agent rituximab; proteasome inhibitor (bortezomib) and terminal complement blockade (eculizumab) are used off-label at experienced centers.

Chronic rejection / chronic allograft dysfunction

• No therapy reverses established interstitial fibrosis, cardiac allograft vasculopathy, or bronchiolitis obliterans. Manage cardiovascular risk and proteinuria, minimize nephrotoxins, and plan retransplantation as definitive care.

Complications of rejection itself

• Graft loss — the endpoint of untreated or refractory rejection; hyperacute rejection with a thrombosed, necrotic graft is a surgical emergency.
• Cardiac allograft vasculopathy: diffuse concentric intimal proliferation in a denervated heart, so ischemia presents as heart failure, arrhythmia, or sudden death rather than angina — a genuine emergency.
• Bronchiolitis obliterans (CLAD): obstructive physiology signaled by a persistent fall in FEV1.
• Transplant glomerulopathy / IF-TA: rising creatinine with proteinuria.

Complications of immunosuppression

• Opportunistic infection, stratified by time: nosocomial and surgical-site infection in the first month; CMV, Pneumocystis jirovecii, BK polyomavirus, and reactivated TB or endemic fungi in months 1–6; community-acquired pathogens later. Per AST/IDSA practice, trimethoprim-sulfamethoxazole PJP prophylaxis and CMV prophylaxis or surveillance are standard. Febrile neutropenia, hypoxemic PJP pneumonia, and CNS infection are emergencies.
• Calcineurin inhibitor nephrotoxicity: afferent arteriolar vasoconstriction → creatinine rise with striped interstitial fibrosis; also hypertension, hyperkalemia, hypomagnesemia, and tremor. It mimics rejection — hence biopsy.
• Drug-specific signatures: tacrolimus → neurotoxicity and post-transplant diabetes; cyclosporine → gingival hyperplasia and hirsutism; mycophenolate → diarrhea, cytopenias, teratogenicity; azathioprine → myelosuppression, catastrophic with allopurinol or low TPMT activity; sirolimus → impaired wound healing, oral ulcers, proteinuria, and pneumonitis; corticosteroids → Cushingoid habitus, osteoporosis, cataracts, hyperglycemia.
• Calcineurin inhibitor–associated thrombotic microangiopathy: schistocytes, thrombocytopenia, falling graft function — emergency, treat by withdrawing the offending agent.
• Post-transplant lymphoproliferative disorder: EBV-driven B-cell proliferation under T-cell suppression, presenting as adenopathy, a graft mass, or B symptoms; first step is reduction of immunosuppression.
• Malignancy surveillance: cutaneous squamous cell carcinoma predominates; lifelong photoprotection and skin examination.
• Cytokine release / anaphylaxis with depleting antibody induction.

• Timing is the diagnosis: minutes-to-hours = hyperacute (pre-formed antibody, type II hypersensitivity, complement-mediated thrombosis); days-to-months = acute (cellular is type IV, T-cell mediated); months-to-years = chronic (fibrosis and intimal proliferation).
• The single best next step for unexplained graft dysfunction is allograft biopsy — after checking calcineurin inhibitor trough and screening for CMV and BK. Do not empirically pulse steroids before you know whether you are treating rejection, CNI toxicity, or BK nephropathy; treating BK with more immunosuppression makes it worse.
• C4d on peritubular capillaries = antibody-mediated rejection; a dense lymphocytic tubulointerstitial infiltrate with tubulitis = acute cellular rejection. AMR is relatively steroid-resistant and needs plasmapheresis with IVIG; ACR responds to pulse methylprednisolone.
• Drug mechanism trap: cyclosporine binds cyclophilin, tacrolimus binds FKBP-12 — both inhibit calcineurin and block NFAT-driven IL-2 transcription. Sirolimus also binds FKBP-12 but inhibits mTOR, not calcineurin. Basiliximab targets CD25 (IL-2 receptor α); belatacept blocks CD80/86 costimulation.
• The association examiners love: azathioprine plus allopurinol → profound myelosuppression (blocked xanthine oxidase metabolism of 6-MP). Check TPMT before azathioprine.
• GVHD versus rejection: GVHD is donor T cells attacking host tissue — classic triad of maculopapular rash (often palms and soles), secretory diarrhea, and cholestatic jaundice after allogeneic HSCT. In an immunodeficient patient receiving non-irradiated blood products, the answer is transfusion-associated GVHD; prevention is irradiation of cellular products, not leukoreduction alone.
• Non-adherence is the most commonly tested modifiable risk factor — the young adult with an undetectable tacrolimus level and rising creatinine.
• Common distractor: chronic rejection is not reversible with augmented immunosuppression; the definitive answer is supportive care and retransplant evaluation.