Immunological Response to Foreign Tissue
Transplanted tissue contains molecular components of the donor’s immune system, known as the major histocompatibility complex (MHC), coupled with antigen presenting cells (APCs), which interact with the host’s immune system. Donor APCs, with the help of MHCs, present peptides (sections of proteins) derived from the transplanted tissue to specialized receptors, called CB8 receptors, on certain T cells (white blood cells involved in cellular immunity) of the host. The host’s T cells recognize that the peptide is foreign and begin traveling through the body in search of cells that contain this peptide.
The host’s T cells are now “activated” and programmed to destroy the cells of the transplanted tissue. As the activated T cells travel, they secrete inflammatory cytokines that serve to recruit and activate additional T cells to help destroy the foreign cells. Importantly, these cytokines stimulate a particularly aggressive class of T cells, called Th17 cells, as well. This process culminates in the initiation of an inflammatory storm that triggers the host’s immune system to mount a full-fledged assault against the transplanted tissue.