Step 1 topics → Immunology → Transplant Rejection (Hyperacute, Acute, Chronic, GVHD)

Transplant Rejection (Hyperacute, Acute, Chronic, GVHD)

USMLE Step 1 trap: Confuses hyperacute rejection timing with acute rejection, missing the minutes-to-hours onset from pre-formed antibodies. Hyperacute rejection occurs within minutes to hours of transplantation due to pre-formed recipient antibodies against donor HLA or ABO antigens activating complement and causing thrombotic occlusion of graft vessels.

Transplant rejection is one of those topics where knowing the mechanism cold is the only way to get the question right — because USMLE Step 1 rarely just asks you to name the type. Instead, it gives you a clinical scenario (timing, biopsy finding, treatment used) and makes you work backward. The four rejection types — hyperacute, acute cellular, acute humoral, and chronic — plus GVHD each have a distinct mechanism, time course, histology, and treatment. Get any one of those details wrong and you'll pick the wrong answer confidently.

The exam tests this from multiple angles: pure recall of timing and mechanism, application of treatment logic (what would you do next?), and passage interpretation where a biopsy description or lab finding is your only clue. The tricky part is that acute cellular and acute humoral rejection overlap in timing but diverge completely in mechanism and treatment — and students routinely blur them. Similarly, GVHD flips the direction of immune attack relative to regular rejection, which trips up students who haven't drilled that distinction.

The highest-yield misconceptions here are timing confusion (hyperacute is minutes, not days), treatment mix-ups between cellular and humoral acute rejection, and reversing who attacks whom in GVHD. USMLE Step 1 loves to exploit all three. Chronic rejection is often the 'default wrong answer' when students don't recognize a specific mechanism — but it has its own distinct vasculopathic pathology that's not just 'repeated acute rejection.' Build each type as a separate mental model and you'll navigate these questions cleanly.

From real student decks
81%have cards covering this topic
70%have mature cards

Common misconceptions

Common mistake
Wrong: Hyperacute rejection occurs days after transplant like acute rejection.
Right: Hyperacute rejection occurs within minutes to hours of transplantation due to pre-formed recipient antibodies against donor HLA or ABO antigens activating complement and causing thrombotic occlusion of graft vessels.
Hyperacute rejection happens within minutes to hours of reperfusion — you can watch the kidney turn mottled and cyanotic on the operating table. This is because the recipient already has pre-formed antibodies (from prior sensitization via pregnancy, transfusion, or previous transplant) that immediately bind donor HLA or ABO antigens, activate complement, and trigger thrombotic occlusion. This is fundamentally different from acute rejection, which requires time for a de novo immune response to develop. If the question says 'days to weeks,' think acute; if it says 'minutes to hours' or 'immediately after reperfusion,' think hyperacute.
Common mistake
Wrong: Acute cellular rejection is treated with plasmapheresis and IVIG like antibody-mediated rejection.
Right: Acute cellular rejection (T cell-mediated, occurring weeks to months post-transplant) is treated with high-dose corticosteroids and anti-thymocyte globulin, not plasmapheresis.
Plasmapheresis targets antibodies — so it makes sense for antibody-mediated (humoral) rejection, not cellular rejection. Acute cellular rejection is driven by recipient T cells infiltrating and attacking the graft, so treatment is aimed at suppressing T cell activity: high-dose corticosteroids (first line) and anti-thymocyte globulin (ATG) for steroid-resistant cases. When you see 'lymphocytic infiltrate on biopsy' and 'weeks post-transplant,' the treatment answer is steroids/ATG, not plasmapheresis. Mixing these up reflects treating the wrong effector arm.
Common mistake
Wrong: Chronic rejection is simply repeated acute rejection episodes.
Right: Chronic rejection is a distinct process driven by both immune (anti-donor antibodies, T cells) and non-immune factors causing progressive intimal smooth muscle proliferation (transplant vasculopathy) and fibrosis, which is largely irreversible and not responsive to immunosuppression.
Chronic rejection is not a slow version of acute rejection — it's a completely different pathologic process. The hallmark is transplant vasculopathy: intimal smooth muscle proliferation causing progressive luminal narrowing, leading to ischemia and fibrosis of the graft over months to years. Both immune factors (donor-specific antibodies, chronic T cell activity) and non-immune factors (hypertension, hyperlipidemia, drug toxicity) drive it. Because the fibrosis is established and the vasculopathy is structural, additional immunosuppression doesn't reverse it. Think of it as a scarring process, not an inflammatory one you can turn off.
Common mistake
Wrong: GVHD occurs when the host immune system attacks the donor graft.
Right: GVHD occurs when donor T cells (in the graft) recognize host tissues as foreign and attack them, classically targeting skin, liver, and GI tract; it is the reverse of typical rejection.
In GVHD, the attack goes graft-to-host — donor T cells in the transplanted tissue recognize the recipient's HLA antigens as foreign and mount an immune attack against the host. This is the opposite of typical rejection (where the host attacks the graft). The classic targets are skin (rash), liver (elevated LFTs, jaundice), and GI tract (secretory diarrhea). It occurs most often after allogeneic bone marrow transplant, where large numbers of donor T cells are transferred. Whenever the question involves a bone marrow or stem cell transplant recipient developing a rash, diarrhea, and liver abnormalities, think GVHD — donor attacking host.
Common mistake
Gap: Missing C4d staining as the diagnostic hallmark of antibody-mediated acute rejection
Antibody-mediated (humoral) acute rejection is diagnosed by the presence of C4d deposits in peritubular capillaries on biopsy, reflecting complement activation by donor-specific antibodies, and is treated with plasmapheresis, IVIG, and rituximab.
C4d is a complement split product that covalently binds to endothelium after complement activation — it's essentially a footprint left behind by antibody-mediated complement activation. On biopsy of a rejecting kidney, C4d staining in peritubular capillaries is the diagnostic hallmark of antibody-mediated (humoral) acute rejection, distinguishing it from cellular rejection even when both occur in the same time window. If you see C4d on a biopsy description, the treatment is plasmapheresis (to remove circulating donor-specific antibodies), IVIG (to modulate remaining antibody effect), and rituximab (to deplete B cells). Missing C4d as a clue means you'll misclassify the rejection type and pick the wrong treatment.
Free Deck audit

See if your Anki deck covers this topic.

Upload your deck →
Guided session

Stuck on this? An AI tutor that probes your understanding.

Start a session →

What the exam tests

  1. Hyperacute rejection: Know that it occurs within minutes to hours of perfusion (not days), is caused by pre-formed recipient antibodies against donor HLA or ABO antigens, activates complement, and causes immediate thrombotic occlusion — and that it's prevented by crossmatch testing before transplant.
  2. Acute cellular rejection: Know that it occurs weeks to months post-transplant, is T cell-mediated, shows lymphocytic infiltration on biopsy, and is treated with high-dose corticosteroids and anti-thymocyte globulin — not plasmapheresis.
  3. Acute humoral (antibody-mediated) rejection: Know that it is diagnosed by C4d deposits in peritubular capillaries on biopsy (reflecting complement activation by donor-specific antibodies) and is treated with plasmapheresis, IVIG, and rituximab.
  4. Chronic rejection: Know that it is a distinct fibroproliferative process (transplant vasculopathy with intimal smooth muscle proliferation and fibrosis), driven by both immune and non-immune factors, largely irreversible, and not responsive to increased immunosuppression — it is not simply accumulated acute episodes.
  5. GVHD: Know that donor T cells (not host immune cells) attack host tissues, that the classic triad of target organs is skin, liver, and GI tract, and that this is the reverse of host-versus-graft rejection — occurring most commonly after allogeneic bone marrow transplant.

Can you avoid these mistakes?

A kidney transplant recipient's new kidney becomes cyanotic and mottled within 20 minutes of reperfusion in the OR. What is the mechanism, and what test would have prevented this?
A patient is 6 weeks post-renal transplant and presents with rising creatinine. Biopsy shows dense lymphocytic infiltrate in the graft tubules with no C4d staining on peritubular capillaries. What type of rejection is this, and what is the first-line treatment?
A different patient at 8 weeks post-transplant also has rising creatinine, but the biopsy shows C4d deposits in peritubular capillaries and circulating donor-specific antibodies are detected. How does the treatment differ from the previous patient, and why?
A patient who received an allogeneic bone marrow transplant 3 weeks ago develops a maculopapular rash, profuse watery diarrhea, and jaundice. What is the diagnosis, who is attacking whom, and what organs are classically involved?

Related topics

Type II Hypersensitivity (Antibody vs Fixed Antigen) Type IV Hypersensitivity (T Cell / Delayed) Thymus and T Cell Maturation Spleen Architecture and Function Lymph Node and MALT Architecture Innate Immune Cells Overview

See how your Anki deck covers this topic.

Upload your deck for a free audit →