Transplant rejection questions are some of the most “pattern-recognition” heavy immunology items on Step 1: if you can quickly map timing + mechanism + histology, you’ll get the answer in seconds. The trick is that “acute rejection” can mean two different immune mechanisms (cellular vs antibody-mediated), and chronic rejection can look like accelerated atherosclerosis of the graft. Let’s make the whole framework feel automatic.
The Big Picture: Timing Is Your First Clue
Classic rejection timeline (high-yield)
| Type | When? | Main mechanism | Key pathology buzzwords | Typical outcome |
|---|---|---|---|---|
| Hyperacute | Minutes–hours | Preformed antibodies (anti-ABO, anti-HLA) → complement | Widespread thrombosis, ischemic necrosis, “no flow” | Graft fails immediately |
| Acute | Days–weeks (or months if immunosuppression reduced) | T cells (most common) ± newly formed antibodies | Endothelitis (cellular), vasculitis/C4d (antibody) | Often reversible with treatment |
| Chronic | Months–years | Chronic T-cell–mediated inflammation ± antibodies | Graft arteriosclerosis, interstitial fibrosis, tubular atrophy | Progressive, usually irreversible |
USMLE move: If the stem says “immediately after reperfusion” → hyperacute. If it’s “weeks after transplant with rising creatinine” → acute. If it’s “years later with progressive decline” → chronic.
Hyperacute Rejection (Type II Hypersensitivity Vibes)
Definition (Step wording)
Hyperacute rejection is immediate graft failure caused by preformed recipient antibodies against donor antigens (usually ABO or HLA).
Pathophysiology
- Recipient already has circulating IgG (or IgM) against donor ABO/HLA.
- Sources: prior transfusions, pregnancies, previous transplant, or natural ABO antibodies.
- Antibodies bind endothelium of graft vessels → complement activation.
- Endothelial injury → platelet activation + coagulation cascade → widespread thrombosis.
- Result: ischemia → graft becomes cyanotic/mottled and fails.
High-yield association: This is essentially a type II hypersensitivity mechanism (antibody-mediated with complement).
Clinical presentation
- Minutes to hours after transplant
- No immediate graft function
- Often described as:
- Kidney: no urine output, rapidly rising creatinine
- Heart: immediate pump failure
- Graft may look blue/black due to ischemia
Diagnosis
- Clinical timing + failure is usually enough.
- Confirmation:
- Evidence of anti-donor antibodies (crossmatch positive pre-op, if missed)
- Pathology: thrombosis and neutrophils in vessels with fibrinoid necrosis
Treatment
- Prevention is everything
- Proper ABO typing
- Crossmatch and panel reactive antibody (PRA) screening
- Once it occurs: remove the graft (generally not salvageable)
First Aid cross-reference (what it usually emphasizes)
- Hyperacute: preformed anti-donor antibodies → complement → thrombosis → necrosis
- Classic timing: minutes to hours
Acute Rejection (Two Mechanisms You Must Separate)
Acute rejection is common, often treatable, and shows up in question stems as “a few weeks after transplant.”
Definition
Acute rejection is graft dysfunction occurring days to weeks after transplant (or later if immunosuppression is reduced), mediated by:
- T cells (acute cellular rejection) and/or
- Antibodies (acute antibody-mediated rejection)
Acute Cellular Rejection (T-cell–mediated; Type IV)
Pathophysiology
- Recipient T cells recognize donor antigens:
- Direct allorecognition: recipient T cells recognize donor MHC on donor APCs
- Indirect allorecognition: recipient APCs present donor peptides on recipient MHC
- Effector mechanisms:
- CD8+ cytotoxicity
- CD4+ cytokines → inflammation, macrophage recruitment
- Targets: tubules and interstitium (kidney), bile ducts (liver), myocardium (heart)
Clinical presentation (classic)
- Days–weeks post-transplant
- Signs of graft dysfunction
- Kidney: rising creatinine, decreased urine output, tenderness over graft
- Liver: cholestatic labs (ALP, bilirubin), transaminitis
- Heart: arrhythmias, heart failure symptoms (often screened via biopsy)
Histology / buzzwords
- Dense lymphocytic infiltrate
- Endothelitis (lymphocytes invading vessel endothelium) is a classic clue in solid organ acute rejection.
Diagnosis
- Often requires biopsy of the graft.
- Rule out non-immune causes (dehydration, drug toxicity, obstruction) in real life—Step will cue you with timing + biopsy.
Treatment
- High-dose corticosteroids (first-line)
- If steroid-resistant: anti–T-cell therapy (e.g., antithymocyte globulin)
- Optimize maintenance immunosuppression:
- Calcineurin inhibitors (tacrolimus/cyclosporine), antiproliferatives (mycophenolate/azathioprine), mTOR inhibitors (sirolimus), steroids
First Aid-style anchor: Acute cellular rejection = T cells + endotheliitis + treat with steroids
Acute Antibody-Mediated Rejection (Humoral)
Pathophysiology
- Recipient develops new antibodies against donor HLA after transplant (or has low-level antibodies missed by screening).
- Antibodies bind graft endothelium → complement activation → vascular injury.
Histology / buzzwords
- Necrotizing vasculitis
- Thrombosis
- C4d deposition in peritubular capillaries (kidney) is a classic marker of complement activation in antibody-mediated rejection.
Diagnosis
- Biopsy with vascular injury + C4d positivity
- Detect donor-specific antibodies (DSA) in serum
Treatment
- Plasmapheresis (remove antibodies)
- IVIG
- Often add rituximab (anti-CD20) or other antibody-targeted approaches (center-specific)
High-yield distinction: Acute rejection can be cellular (steroids) vs humoral (plasmapheresis/IVIG)—Step questions love to test this via biopsy clues (endothelitis vs C4d).
Chronic Rejection (The “Accelerated Atherosclerosis” Pattern)
Definition
Chronic rejection is progressive graft dysfunction occurring months to years after transplant due to chronic immune-mediated injury leading to fibrosis and vascular narrowing.
Pathophysiology
- Persistent low-grade T-cell–mediated inflammation with cytokine-driven smooth muscle proliferation
- Antibodies can contribute, but the hallmark is chronic vascular changes
- Leads to:
- Graft arteriosclerosis (intimal thickening)
- Interstitial fibrosis
- Parenchymal atrophy
Clinical presentation
- Slow, progressive decline in graft function
- Often no dramatic systemic symptoms
- Examples:
- Kidney: gradually rising creatinine, hypertension, proteinuria
- Heart: progressive ischemic cardiomyopathy from graft vasculopathy
- Lung: bronchiolitis obliterans (important in Step 2-style stems)
Histology / buzzwords
- Intimal thickening and narrowing of graft vessels (“graft arteriosclerosis”)
- Interstitial fibrosis and parenchymal atrophy
- Think: “chronic inflammation → scarring”
Diagnosis
- Clinical course + biopsy showing chronic vascular/fibrotic changes
- Important: exclude recurrent disease and medication toxicity (Step may hint at calcineurin inhibitor nephrotoxicity vs chronic rejection—chronic rejection points to vascular narrowing + fibrosis)
Treatment
- Limited response to immunosuppression
- Manage complications; often leads to retransplantation
First Aid-style anchor: Chronic rejection = months–years, arteriosclerosis + fibrosis, irreversible
How to Crush USMLE Questions: A 10-Second Algorithm
- Look at timing
- Minutes–hours → hyperacute
- Days–weeks (or after reducing immunosuppression) → acute
- Months–years → chronic
- Look at mechanism clues
- Preformed Ab, complement, thrombosis → hyperacute
- Lymphocytes + endotheliitis → acute cellular
- C4d + vasculitis/thrombosis → acute antibody-mediated
- Intimal thickening/fibrosis → chronic
- Pick treatment
- Hyperacute → prevent / remove graft
- Acute cellular → steroids
- Acute antibody-mediated → plasmapheresis + IVIG
- Chronic → supportive / retransplant
High-Yield Mini-Table: What the Stem Will Literally Say
| Stem phrase | Most likely answer |
|---|---|
| “Immediately after transplant, graft becomes cyanotic; widespread thrombosis” | Hyperacute rejection |
| “Weeks after transplant, biopsy shows lymphocytes in vessel wall (endothelitis)” | Acute cellular rejection |
| “Acute rejection with C4d deposition / donor-specific antibodies” | Acute antibody-mediated rejection |
| “Years later, progressive graft failure; concentric intimal thickening” | Chronic rejection |
First Aid Cross-References (How It’s Usually Organized)
In First Aid, transplant rejection is typically grouped under Immunology → Hypersensitivity/Transplantation concepts with these core one-liners:
- Hyperacute: preformed antibodies → complement → thrombosis/necrosis
- Acute: T cells (± antibodies) → treat with immunosuppression
- Chronic: vascular smooth muscle proliferation → arteriosclerosis/fibrosis
If your copy mentions type II vs type IV hypersensitivity, map:
- Hyperacute → Type II
- Acute cellular → Type IV
- Chronic → mixed chronic immune injury (often framed around chronic inflammation + vascular changes)
Common Pitfalls (What Tricks Students)
- “Acute rejection” ≠ always T-cell only. If they give C4d or donor-specific antibodies, think humoral.
- Chronic rejection is not “late acute rejection.” It’s a distinct pathology: arteriosclerosis + fibrosis, usually irreversible.
- Hyperacute is preventable with crossmatching; when it happens, it’s dramatic and immediate.
Rapid Review (One-Liners You Should Be Able to Say Out Loud)
- Hyperacute: minutes–hours, preformed anti-ABO/HLA, complement → thrombosis, graft dies.
- Acute cellular: days–weeks, T cells, endothelitis, treat with steroids.
- Acute humoral: days–weeks, new antibodies, C4d, treat with plasmapheresis + IVIG.
- Chronic: months–years, graft arteriosclerosis + fibrosis, progressive failure, often retransplant.