Overview of Transplantation

ByMartin Hertl, MD, PhD, Rush University Medical Center
Reviewed/Revised Aug 2022
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Transplants may be

  • The patient’s own tissue (autografts; eg, bone, bone marrow, and skin grafts)

  • Genetically identical (syngeneic [between monozygotic twins]) donor tissue (isografts)

  • Genetically dissimilar donor tissue (allografts, or homografts)

  • Rarely, grafts from a different species (xenografts, or heterografts)

Transplanted tissue may be

Tissues may be grafted to an anatomically normal site (orthotopic; eg, heart transplants) or abnormal site (heterotopic; eg, a kidney transplanted into the iliac fossa).

Almost always, transplantation is done to replace or restore the function of an end-stage diseased organ, thereby restoring an essential function and improving patient survival. However, some procedures (eg, hand, larynx, tongue, uterus, facial transplantation) enhance the quality of life but do not improve survival and have significant risks related to surgery and immunosuppression. Some of these procedures are highly specialized and done infrequently but are no longer considered experimental.

Clinical transplantation uses allografts from donors who are

  • Living and related to the patient

  • Living and unrelated to the patient

  • Deceased

Living donors are often used for kidney and hematopoietic stem cell transplants, less frequently for segmental liver transplants, and rarely for pancreas and lung transplants. Use of deceased-donor organs (from heart-beating or non–heart-beating donors) has helped reduce the disparity between organ demand and supply; however, demand still far exceeds supply, and the number of patients waiting for organ transplants continues to grow.

Some research is ongoing in transplantation of non-human organs, such as heart, kidney and liver transplantation. Despite progress in the field, clinical implementation is still delayed, mainly due to insurmountable rejection, but also due to critical functional differences between xenografts and human tissue.

Graft rejection and graft-vs-host disease

All allograft recipients are at risk of graft rejection; the recipient’s immune system recognizes the graft as foreign and seeks to destroy it. Recipients of grafts containing immune cells (particularly bone marrow, intestine, and liver) are at risk of graft-vs-host disease, in which the donor immune cells attack recipient tissue. Risk of these complications is minimized by pretransplantation screening and immunosuppressive therapy during and after transplantation.

Organ allocation

In the US, an organ allocation system has been implemented for livers and other organs. Under this system, transplant candidates receive an organ based on medical urgency and the distance between the donor hospital and transplant hospitals. This system replaces the previous system that was based primarily on the geographic boundaries of donation service areas and transplant regions. Now, livers and other organs from all adult deceased donors are offered first to compatible transplant candidates who have the most urgent need (Status 1A and 1B) and are listed at transplant hospitals within 500 nautical miles of the donor hospital. After offers are made to the most urgent candidates, organs are offered to candidates at hospitals within distances of 150, 250, and 500 nautical miles of the donor hospital. In kidney allocation, wait time remains a challenge, and, for all organs, pediatric organs are primarily allocated to pediatric candidates first.

Pretransplantation Screening

Before the risk and expense of transplantation are undertaken and scarce donor organs are committed, medical teams screen potential recipients for medical and nonmedical factors that may affect the likelihood of success. Donors undergo medical screening, psychological screening, and laboratory studies, including tests to evaluate risk of infection.

Tissue compatibility

In pretransplantation screening, recipients and donors are tested for

Recipients are tested for

  • Presensitization to donor antigens

HLA tissue typing is most important for the following:

Transplantation of the following typically occurs urgently, often before HLA tissue typing can be completed, so the role of matching for these organs is less well-established:

HLA tissue typing of peripheral blood or lymph node lymphocytes is used to match the most important known determinants of histocompatibility in the donor and recipient. More than 1250 alleles determine 6 HLA antigens (HLA-A, -B, -C, -DP, -DQ, -DR), so matching is a challenge; eg, in the US, only 2 of 6 antigens on average are matched in kidney donors and recipients. Matching of as many HLA antigens as possible significantly improves functional survival of grafts from living related kidney and hematopoietic stem cell donors; HLA matching of grafts from unrelated donors also improves survival, although much less so because of multiple undetected histocompatibility differences. Better immunosuppressive therapy has expanded eligibility for transplantation; HLA mismatches do not automatically disqualify patients for transplantation because immunosuppressive therapy has become more effective.

ABO compatibility and HLA compatibility are important for graft survival. ABO mismatches can precipitate hyperacute rejection of vascularized grafts (eg, kidney, heart), which have ABO antigens on the endothelial surfaces. Presensitization to HLA and ABO antigens results from prior blood transfusions, transplantations, or pregnancies and can be detected with serologic tests or, more commonly, with a lymphocytotoxic test using the recipient’s serum and donor’s lymphocytes in the presence of complement. A positive cross-match indicates that the recipient’s serum contains antibodies directed against ABO or class I HLA antigens in the donor; it is an absolute contraindication to transplantation, except possibly in infants (up to age 14 months) who have not yet produced isohemagglutinins.

High-dose IV immune globulin and plasma exchange have been used to suppress HLA antibodies and facilitate transplantation when a more compatible graft is not available. Costs are high, but midterm outcomes are encouraging and appear similar to those in unsensitized patients.

Even a negative cross-match does not guarantee safety; when ABO antigens are compatible but not identical (eg, donor O and recipient A, B, or AB), hemolysis is a potential complication due to antibody production by transplanted (passenger) donor lymphocytes.

Although matching for HLA and ABO antigens generally improves graft survival, patients who are not White are disadvantaged because

  • Organ donation is less common among people who are not White, and, thus, the number of potential donors who are not White is limited.

  • End-stage renal disease is more common among people of African ancestry, and, thus, the need for organs is greater.

  • Patients who are not White may have HLA polymorphisms that are different from those of donors who are White, a higher rate of presensitization to HLA antigens, and a higher incidence of blood types O and B.


Donor and recipient exposure to common infectious pathogens and active as well as latent infections must be detected before transplantation to minimize risk of transmitting infection from the donor and risk of worsening or reactivating existing infection in the recipient (due to use of immunosuppressants).

This screening usually includes the history and tests for

  • Cytomegalovirus (CMV)

  • Epstein-Barr virus

  • Hepatitis B virus

  • Hepatitis C viruses

  • Herpes simplex virus (HSV)

  • HIV

  • Mycobacterium tuberculosis

  • Varicella-zoster virus

  • West Nile virus, if exposure is suspected

Positive findings may require pre- or posttransplantation antiviral treatment (eg, for CMV infection or hepatitis B or hepatitis C) or anti-tuberculosis treatment or, in the case of active infection, contraindicate transplantation until the infection is controlled (eg, if HIV with AIDS is detected).

Recipient contraindications to transplantation

Absolute contraindications to transplantation include the following:

  • Clinically active infection, except possibly infection in the recipient if it is confined to the organ being replaced (eg, liver abscesses)

  • Cancer (except hepatocellular carcinoma confined to the liver, non-melanoma skin cancers, and certain neuroendocrine tumors)

  • A positive cross-match identified by lymphocytotoxic testing

Relative contraindications include the following:

  • Age > 70

  • Poor functional or nutritional status (including severe obesity)

  • HIV infection

  • Multiorgan insufficiency

Psychologic and social factors also play an important role in success of transplantation. For example, people with a substance use disorder s or those who are psychologically unstable are less likely to firmly adhere to the necessary lifelong regimen of treatments and follow-up visits.

Eligibility decisions for patients with relative contraindications differ by medical center. HIV infection is no longer considered an absolute contraindication because antivirals and immunosuppressants are usually well-tolerated by and effective in transplant recipients who require them.

Posttransplantation Immunosuppression

Immunosuppressants control graft rejection and are primarily responsible for the success of transplantation (see table Immunosuppressants Used to Treat Transplant Rejection). However, they suppress all immune responses and contribute to many posttransplantation complications, including development of cancer, acceleration of cardiovascular disease, and even death due to overwhelming infection.

Immunosuppressants must usually be continued long after transplantation, but initially high doses can be reduced a few weeks after the procedure, and low doses can be continued indefinitely unless rejection occurs. Further reduction of immunosuppressant doses long after transplantation and protocols for inducing tolerance of donor organs are under study.


A high dose of corticosteroids is usually given at the time of transplantation, then the dose is reduced gradually to a maintenance dose, which is given indefinitely. Several months after transplantation, corticosteroids can be given on alternate days; this regimen helps prevent growth restriction in children. If rejection occurs, high doses are reinstituted.

Calcineurin inhibitors

Purine metabolism inhibitors


Immunosuppressive immunoglobulins

Examples are

  • Antilymphocyte globulin (ALG)

Both are fractions of animal antisera directed against human cells:

  • Lymphocytes: ALG

  • Thymus cells: ATG

ALG and ATG suppress cellular immunity while preserving humoral immunity. They are used with other immunosuppressants to allow those drugs to be used in lower, less toxic doses. Use of ALG or ATG to control acute episodes of rejection improves graft survival rates; use at the time of transplantation may decrease rejection incidence and allow calcineurin inhibitors to be started later, thereby reducing toxicity.

Use of highly purified serum fractions has greatly reduced incidence of adverse effects (eg, anaphylaxis, serum sickness, antigen-antibody–induced glomerulonephritis).

Monoclonal antibodies (mAbs)

Monoclonal antibodies directed against T cells provide a higher concentration of anti-T-cell antibodies and fewer irrelevant serum proteins than do ALG and ATG.

Anti–IL-2 receptor monoclonal antibodies

Other drugs


Irradiation of a graft, local recipient tissues, or both can be used to treat kidney transplant rejection when other treatment (eg, corticosteroids and ATG) has been ineffective. Total lymphatic irradiation appears to safely suppress cellular immunity, at first by stimulation of suppressor T cells and later possibly by clonal deletion of specific antigen-reactive cells. However, because immunosuppressants are now so effective, the need for irradiation is extremely rare.

Future therapies

Protocols and agents to induce graft antigen-specific tolerance without suppressing other immune responses are being sought. Two strategies are promising:

  • Blockade of T-cell costimulatory pathways using a cytotoxic T lymphocyte–associated antigen 4 (CTLA-4)-IgG1 fusion protein


Posttransplantation Complications

Complications include the following:

  • Rejection

  • Infection

  • Renal insufficiency

  • Cancer

  • Atherosclerosis


Rejection of solid organs may be hyperacute, accelerated, acute, or chronic (late). These categories can be distinguished histopathologically and approximately by the time of onset. Symptoms vary by organ (see table Manifestations of Transplant Rejection by Category).

Hyperacute rejection has the following characteristics:

  • Occurs within 48 hours of transplantation

  • Is caused by preexisting complement-fixing antibodies to graft antigens (presensitization)

  • Is characterized by small-vessel thrombosis and graft infarction

It has become rare (1%) as pretransplantation screening has improved. No treatment is effective except graft removal.

Accelerated rejection has the following characteristics:

  • Occurs 3 to 5 days after transplantation

  • Is caused by preexisting noncomplement-fixing antibodies to graft antigens

  • Is characterized histopathologically by cellular infiltrate with or without vascular changes

Accelerated rejection is also rare. Treatment is with high-dose pulse corticosteroids or, if vascular changes occur, antilymphocyte preparations. Plasma exchange, which may clear circulating antibodies more rapidly, has been used with some success.

Acute rejection is graft destruction after transplantation and has the following characteristics:

  • Occurs later, about 5 days after transplantation, and unlike hyperacute and accelerated rejection, acute rejection is mediated by a de novo anti-graft T-cell response, not by preexisting antibodies

  • Is caused by a T cell–mediated delayed hypersensitivity reaction to allograft histocompatibility antigens

  • Is characterized by mononuclear cellular infiltration, with varying degrees of hemorrhage, edema, and necrosis but with vascular integrity usually maintained (although vascular endothelium appears to be a primary target)

Acute rejection accounts for about half of all rejection episodes that occur within 10 years. Acute rejection is often reversed by intensifying immunosuppressive therapy (eg, with pulse corticosteroids, ALG, or both). After rejection reversal, severely damaged parts of the graft heal by fibrosis, the remainder of the graft functions normally, immunosuppressant doses can be reduced to very low levels, and the allograft can survive for long periods.

Chronic rejection is graft dysfunction, often without fever. It has the following characteristics:

  • Typically occurs months to years after transplantation but sometimes within weeks

  • Has multiple causes, including early or late antibody-mediated rejection, periprocedural ischemia and reperfusion injury, drug toxicity, infection, and vascular factors (eg, hypertension, hyperlipidemia)

  • Is characterized pathologically by proliferation of neointima consisting of smooth muscle cells and extracellular matrix (transplantation atherosclerosis), which gradually and eventually occludes vessel lumina, resulting in patchy ischemia and fibrosis of the graft (in liver transplant recipients, paucity of bile ducts can be the leading pathologic feature)



Transplant patients become vulnerable to infections because of

  • Use of immunosuppressants

  • Secondary immunodeficiencies that accompany organ failure

  • Surgery

Rarely, a transplanted organ is the source of infection (eg, cytomegalovirus [CMV]).

The most common sign is fever, often without localizing signs. Fever can also be a symptom of acute rejection but is usually accompanied by signs of graft dysfunction. If these signs are absent, the approach is similar to that for other fever of unknown origin; timing of symptoms and signs after transplantation helps narrow the differential diagnosis.

In the first month after transplantation, most infections are caused by the same hospital-acquired bacteria and fungi that infect other surgical patients (eg, Pseudomonas species causing pneumonia, gram-positive bacteria causing wound infections). The greatest concern with early infection is that organisms can infect a graft or its vascular supply at suture sites, causing mycotic aneurysms or dehiscence.

Opportunistic infections occur 1 to 6 months after transplantation (for treatment, see elsewhere in THE MANUAL). Infections may be bacterial (eg, listeriosis, nocardiosis), viral (eg, due to BK virus [a common opportunistic polyomavirus in kidney transplant recipients], CMV, Epstein-Barr virus, varicella-zoster virus, or hepatitis B or hepatitis C virus), fungal (eg, aspergillosis, cryptococcosis, Pneumocystis jirovecii infection), or parasitic (eg, strongyloidiasis, toxoplasmosis, trypanosomiasis, leishmaniasis). Historically, many of these infections were associated with the use of high-dose corticosteroids.

Risk of infection returns to baseline in about 80% of patients after 6 months. About 10% develop complications of early infections, such as viral infection of the graft, metastatic infection (eg, CMV retinitis, colitis), or virus-induced cancers (eg, hepatitis and subsequent hepatocellular carcinoma, human papillomavirus and subsequent basal cell carcinoma). Others develop chronic rejection, require high doses of immunosuppressants (5 to 10%), and remain at high risk of opportunistic infections indefinitely. Risk of infection varies depending on the graft received and is lowest for recipients of kidney allografts and highest for recipients of liver and lung transplants.

P. jirovecii

Inactivated vaccines can be safely given posttransplantation. Risks due to live-attenuated vaccines must be balanced against their potential benefits because clinically evident infection and exacerbation of rejection are possible in immunosuppressed patients, even if blood levels of immunosuppressants are low.

Renal disorders

Glomerular filtration rate (GFR) decreases 30 to 50% during the first 6 months after solid organ transplantation in 15 to 20% of patients. These patients usually also develop hypertension. Incidence is highest for recipients of small-bowel transplants (21%) because high blood levels of immunosuppressants (usually calcineurin inhibitors) are needed to maintain the graft. Incidence is lowest for recipients of heart-lung transplants (7%). Nephrotoxic and diabetogenic effects of calcineurin inhibitors are the most important contributor, but periprocedural renal damage, pretransplantation renal insufficiency, and use of other nephrotoxic drugs also contribute.

After the initial decrease, GFR typically stabilizes or decreases more slowly; nonetheless, mortality risk quadruples in patients progressing to end-stage renal disease requiring dialysis unless subsequent kidney transplantation is done. Renal insufficiency after transplantation may be prevented by early weaning from calcineurin inhibitors, but a safe minimum dose has not been determined.


Long-term immunosuppression increases incidence of virus-induced cancer, especially squamous and basal cell carcinoma, lymphoproliferative disorders (mainly B-cell non-Hodgkin lymphoma), anogenital (including cervical) and oropharyngeal cancer, and Kaposi sarcoma.

Other complications


Failure to grow, primarily as a consequence of chronic corticosteroid use, is a concern in children. Growth failure can be mitigated by tapering corticosteroids to the minimum dose that does not lead to graft rejection.

Atherosclerosis> 15 years posttransplantation.

Graft-vs-host disease (GVHD) occurs when donor T cells react against recipient self-antigens. GVHD primarily affects hematopoietic stem cell recipients but may also affect liver and small-bowel transplant recipients. It can include inflammatory damage to tissues, especially the liver, intestine, and skin, as well as blood dyscrasia.

Gout can be exacerbated in patients with pre-existing hyperuricemia and/or a history of gout, especially those undergoing kidney or heart transplantation, and may be exacerbated posttransplantation because of drug interactions. Aggressive preoperative treatment of patients with increased risk with uric acid lowering drugs may be helpful in prevention.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

  1. United Network for Organ Sharing: Transplant Trends. https://unos.org/data/transplant-trends/: Provides guidance on organ allocation and comprehensive information on transplantation, including numbers of transplants

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