Clinical Pathology · MU Sofia

30. Transplantation immunity. Rejection of tissue transplants.

Covers how the immune system recognises a transplanted organ as foreign, with the MHC mismatch between donor and patient as the trigger. It sets out the direct cellular and indirect antibody-mediated pathways, then graft-versus-host disease with a bone marrow transplant example. Transplant types are listed, followed by hyperacute, acute and chronic rejection, with a timing and morphology summary and a kidney graft biopsy micrograph.

Inside: 1 section · 1 labelled picture · 15 flashcards · 3 min

Topic 30 of 138 in the MU Sofia syllabus for Clinical Pathology (Medicine, Year 4). The free AceMed note covers it in 1 section, about 509 words (3 min of reading), with 15 flashcards.

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  1. Transplantation immunity. Rejection of tissue transplants

Transplantation immunity. Rejection of tissue transplants

Transplant immunity

When organs are transplanted into the body the immune system recognises this as foreign prompting an immune response.

The thing that actually causes the rejection in transplants is the differing MHC molecules on the donated organ to that of the patient. MHC molecules are codominantly expressed and inherited as haplotypes (one from each parent), this makes each person half identical to each parent, and only 25% likely to have a sibling with the same MHC genetic coding.

Rejection is an adaptive immune response via cellular immunity (killer T-cells) as well as humoral immunity (activated B-cells secreting antibodies).

There are two rejection pathways, direct and indirect.

Direct rejection (cells) pathway:

  • CD8+ T-cells identify foreign MHCI and activate to become killer T-cells which then start killing the transplant

Indirect (Antibodies)

  • CD4+ T-cells identify donor MHC peptides presented on the host's own MHC II and activate, releasing cytokines prompting B-lymphocytes to start releasing antibodies against the transplanted organ.

Graft vs Host

This is the term given to when immune cells (T cells) in the donated tissue attack the host's tissues (via the mechanism explained below).

The symptoms include damage to the liver, skin, mucosa and Gastrointestinal tract as well as the primary lymphatic systems of bone marrow and thymus.

Example: after an allogeneic bone marrow (haematopoietic stem cell) transplant, immunocompetent donor T cells attack the recipient's tissues: skin rash, jaundice from bile duct injury, and diarrhoea from damage to the intestinal mucosa.

Rejection can be acute or chronic depending on if it is in the first 100 days or later.

Types of transplant

Transplants can be:

  • Autograft
  • Tissue comes from the patient themselves
  • Allograft
  • Transplant of an organ or tissue between genetically non-identical members of the same species
  • Isograft
  • Graft where tissue is transplanted from a genetically identical individual (identical twin)
  • Xenograft
  • Transplant from another species (e.g. pig heart valves)
  • Split transplants
  • Organ split and divided between two individuals (e.g. liver between adult and child)
  • Domino transplant
  • In those with cystic fibrosis where both lungs need to be transplanted it is easier to also replace the heart. As the heart is technically still healthy this is then donated to another.

There are three types of Transplant rejection:

  • Hyperacute
  • Caused by preformed antibodies, e.g. blood type incompatibility (Type II hypersensitivity, immediate)
  • Acute
  • T-cell mediated immune response against foreign MHC, occurs in weeks to months.
  • Chronic

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