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How does tissue typing improve the chances that a transplanted organ will be accepted?

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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.

Tissue typing improves the chances of organ acceptance by identifying the degree of compatibility between a donor and a recipient’s human leukocyte antigens (HLA). These antigens, part of the major histocompatibility complex, act as markers that allow the immune system to distinguish "self" from "nonself" antigens.12

When a transplant occurs, the recipient's T lymphocytes recognize foreign HLA markers on the donor tissue as "nonself," triggering a cell-mediated immunity response that leads to graft rejection. By testing the lymphocytes of both parties against a spectrum of typing sera, clinicians can determine how many HLA antigens they share.21

A closer match reduces the likelihood that the recipient's immune system will mount a destructive attack against the transplanted organ. Additionally, a final direct crossmatch between the recipient's serum and donor lymphocytes is performed to ensure the recipient does not already possess preformed antibodies that would cause rapid, irreversible rejection.12