How Organ Transplants Work: Matching, Surgery, and Rejection Prevention

This article is for informational purposes only. Consult a qualified healthcare professional for medical advice, diagnosis, or treatment.

What Is Organ Transplantation?

Organ transplantation is a surgical procedure in which a failing or damaged organ is replaced with a healthy one from a donor. It represents one of medicine's most remarkable achievements — turning end-stage organ failure into a survivable condition. The first successful kidney transplant was performed in 1954 at Brigham Hospital in Boston by Dr. Joseph Murray, who later received the Nobel Prize in Physiology or Medicine. Today, thousands of transplants are performed annually worldwide, covering kidneys, livers, hearts, lungs, pancreases, and small intestines, as well as composite tissue allografts such as hand and face transplants.

In the United States alone, more than 46,000 organ transplants were performed in 2023. However, demand vastly exceeds supply: over 100,000 Americans remain on the national transplant waiting list, and approximately 17 people die each day waiting for an organ.

Types of Organ Donors

Donor Type	Description	Organs Typically Available
Deceased donor (brain death)	Legal brain death while heart still beats; organs perfused until recovery	Heart, liver, kidneys, lungs, pancreas, intestine, corneas, tissue
Deceased donor (DCD — donation after circulatory death)	Cardiac and respiratory function ceases; organs recovered after brief wait	Kidneys, liver, lungs (select centers)
Living related donor	Genetically related individual (parent, sibling, child)	Kidney (single), partial liver lobe
Living unrelated donor	Spouse, friend, or altruistic stranger	Kidney, partial liver lobe

Living donation offers several advantages: planned timing, shorter cold ischemia time (time the organ spends without blood flow), and in the case of related donors, better HLA compatibility. The most common living-donor transplant is the kidney, since healthy individuals can function normally with a single kidney.

Organ Matching: The Science of Compatibility

Successful transplantation depends on minimizing immunological incompatibility between donor and recipient. The immune system recognizes foreign proteins on transplanted organ cells and launches an attack. Several factors are assessed to find the best match:

Blood Type (ABO Compatibility)

ABO blood group antigens are expressed on red blood cells and on vascular endothelium of transplanted organs. Incompatible blood types cause hyperacute rejection within minutes as preformed antibodies bind to donor endothelium. Most transplants require ABO-compatible or, for kidneys and livers, sometimes ABO-identical matching.

Human Leukocyte Antigens (HLA)

HLA molecules (encoded by the Major Histocompatibility Complex, MHC) are the primary targets of immune rejection. Each person inherits two sets of HLA genes — one from each parent — encoding HLA-A, HLA-B, and HLA-DR antigens (the most clinically relevant). The more HLA antigens donor and recipient share, the better the long-term outcome, particularly for kidney transplants. A six-antigen match (0-mismatch) offers the best prognosis. National registries like UNOS (United Network for Organ Sharing) in the US use HLA typing data to optimize donor-recipient matching for deceased-donor organs.

Crossmatch Testing

Before transplantation, a crossmatch test mixes recipient serum with donor lymphocytes to detect preformed donor-specific antibodies (DSAs). A positive crossmatch indicates the recipient has antibodies that will attack the donor organ, and transplantation generally proceeds only after desensitization protocols or is avoided altogether.

The Surgical Procedure

Surgical techniques vary by organ. Key aspects:

Kidney transplant: The donor kidney is typically placed in the recipient's lower pelvis (heterotopic position), not in place of the native kidneys, which are often left in situ. The renal artery and vein are anastomosed to the external iliac vessels, and the ureter is connected to the bladder. Surgery takes 2–4 hours.
Liver transplant: The diseased liver is removed and the donor liver placed in the same anatomical position (orthotopic). Five vascular anastomoses are made: suprahepatic inferior vena cava, infrahepatic inferior vena cava, portal vein, hepatic artery, and bile duct. Surgery takes 6–12 hours.
Heart transplant: The recipient's heart is removed (except for the posterior walls of the atria), and the donor heart is implanted via four anastomoses: aorta, pulmonary artery, left atrium, right atrium. The patient is maintained on cardiopulmonary bypass during the procedure. Surgery takes 4–6 hours.
Lung transplant: Single or bilateral lung transplantation involves anastomosis of bronchus, pulmonary artery, and pulmonary veins. Lung transplants are particularly vulnerable to primary graft dysfunction due to ischemia-reperfusion injury.

Organ Rejection: Types and Mechanisms

Rejection Type	Timing	Mechanism	Treatment
Hyperacute	Minutes to hours post-transplant	Preformed antibodies activate complement, destroy vasculature	Prevention (crossmatch); no effective treatment once started
Acute cellular	Days to months post-transplant	T-cell mediated attack on donor cells	High-dose corticosteroids; antithymocyte globulin
Acute antibody-mediated	Days to months post-transplant	Donor-specific antibodies damage vasculature	Plasmapheresis, IVIG, rituximab
Chronic rejection	Months to years post-transplant	Slow immune and non-immune injury; fibrosis and vascular damage	Optimize immunosuppression; no reliable reversal

Immunosuppression: Preventing Rejection

Recipients must take immunosuppressive medications for life to prevent the immune system from attacking the transplanted organ. Standard regimens typically combine three drug classes:

Calcineurin inhibitors (CNIs): Tacrolimus (FK506) and cyclosporine block T-cell activation by inhibiting the enzyme calcineurin, which is needed to produce interleukin-2 (IL-2), the principal T-cell growth factor. Tacrolimus is now the preferred CNI due to better efficacy. Side effects include nephrotoxicity, hypertension, and neurotoxicity.
Antiproliferative agents: Mycophenolate mofetil (MMF) inhibits de novo purine synthesis, blocking lymphocyte proliferation. Azathioprine is an older alternative.
Corticosteroids: Prednisone provides broad anti-inflammatory effects. Many protocols aim to taper or eliminate steroids over time to reduce long-term side effects (osteoporosis, diabetes, infection susceptibility).
mTOR inhibitors: Sirolimus and everolimus block the mTOR pathway, suppressing T-cell proliferation. Used as adjuncts or CNI-sparing agents.

The major risk of long-term immunosuppression is increased susceptibility to infections (particularly opportunistic infections such as CMV, BK virus, and Pneumocystis) and a higher risk of certain cancers, particularly skin cancers and lymphomas.

Outcomes and Advances

Transplant outcomes have improved dramatically. One-year kidney transplant survival now exceeds 95% for living-donor organs and 92% for deceased-donor organs. Long-term graft survival at 10 years is approximately 65% for deceased-donor kidneys. Chronic rejection and nephrotoxicity from CNIs remain major barriers to even longer graft survival.

Emerging areas include xenotransplantation (transplanting genetically modified pig organs into humans), organ preservation technologies that extend viability with machine perfusion, and tolerance induction protocols that may eventually allow transplant recipients to reduce or eliminate immunosuppression.