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Why can the immune system reject transplanted tissue?

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Immune stimulation by activated helper T cells — immune system Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, T_H1 and T_H2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.

The immune system rejects transplanted tissue because it is designed to distinguish "self" from "nonself" substances. This process is primarily driven by the major histocompatibility complex (MHC), a group of genes that code for cell-surface proteins.12

In humans, these proteins are known as the human leukocyte antigen (HLA) system. Because these molecules are highly diverse, it is rare for two individuals to share the same tissue type. When an organ or tissue is transplanted, the recipient's immune system recognizes the donor's MHC molecules as foreign antigens and initiates an attack.21

Rejection is generally caused by cell-mediated immunity, where T lymphocytes infiltrate and destroy the graft. In some cases, B lymphocytes produce antibodies that contribute to a more rapid, antibody-mediated rejection. To minimize these risks, medical professionals prioritize close tissue matching and the use of immunosuppressive agents to suppress the recipient's immune response.13