Transplant Immunology and Rejection

Core Idea

Transplant rejection occurs when the immune system attacks transplanted tissue. Acute rejection (hours to months) is mediated by T cells recognizing donor MHC-peptide (direct allorecognition) or recipient APCs presenting processed donor antigen (indirect allorecognition). Chronic rejection develops over years as antibodies and T cells target donor antigens. Immunosuppression prevents rejection but increases infection and malignancy risk.

Explainer

You already understand that MHC molecules present peptide fragments on the cell surface for T cell inspection, and that this system is what allows T cells to distinguish self from non-self. Transplant immunology is fundamentally a story about what happens when a recipient's immune system encounters MHC molecules it has never seen before — molecules that look foreign simply because they come from a genetically different individual. MHC genes (called HLA in humans) are the most polymorphic in the entire genome, meaning that any two unrelated people almost certainly carry different versions. This extreme diversity, so beneficial for population-level defense against pathogens, becomes the central obstacle in organ transplantation.

The immune system recognizes donor tissue through two distinct pathways. In direct allorecognition, recipient T cells bind directly to donor MHC molecules on the surface of transplanted cells. This is unusual because T cells are normally trained to recognize peptides presented by self-MHC, but donor MHC molecules are so structurally different that they can activate a surprisingly large fraction of the recipient's T cell repertoire — estimates suggest 1–10% of T cells can respond to a single foreign MHC type, compared to the tiny fraction that responds to any single conventional antigen. In indirect allorecognition, recipient antigen-presenting cells ingest donor proteins (including shed MHC molecules), process them into peptide fragments, and present those fragments on recipient MHC. This second pathway works through the normal antigen presentation machinery you already know, and it becomes increasingly important over time as donor cells are destroyed and their proteins are scavenged.

These recognition events drive the three clinical patterns of rejection. Hyperacute rejection occurs within minutes to hours when preformed recipient antibodies (from prior transfusions, pregnancies, or transplants) bind donor endothelial cells and activate complement, causing immediate vascular destruction — this is now largely prevented by pre-transplant crossmatch testing. Acute rejection develops over days to months as T cells infiltrate the graft; CD8+ cytotoxic T cells directly kill donor cells, while CD4+ T cells coordinate the inflammatory response. Chronic rejection unfolds over months to years through a combination of antibody-mediated vascular damage, persistent T cell activity, and fibrotic remodeling that gradually destroys graft function.

Managing transplant rejection requires deliberately suppressing the very immune responses you have learned are essential for fighting infection. Immunosuppressive drugs like calcineurin inhibitors (cyclosporine, tacrolimus) block T cell activation signaling, while agents like mycophenolate inhibit lymphocyte proliferation. The fundamental trade-off is inescapable: suppressing rejection increases susceptibility to infections and certain cancers, because the same surveillance mechanisms that attack the graft also protect against pathogens and malignant cells. This is why transplant medicine seeks the narrowest effective immunosuppression — enough to prevent rejection, but not so much that the patient becomes defenseless. Regulatory T cells, which you studied in immune tolerance, represent a frontier of research aimed at inducing graft-specific tolerance without broad immunosuppression.