Allorecognition

The response to histoincompatible tissues was designated as allorecognition. Histocompatibility antigens can be recognized by (allo)antibodies and by T cells. The strongest alloreaction is directed against incompatible molecules encoded by the Major Histocompatibility Complex (MHC). This can be explained by the fact that these MHC molecules play a fundamental role in antigen presentation and are thus optimally shaped to be recognized by T cells. Furthermore, they are extremely polymorphic so that two individuals virtually always express a different set of MHC molecules.

Allorecognition at the basis of a rejection of an organ have many points in common with the allorecognition causing GVHD. The response is initiated by the stimulator APC expressing the alloantigens will prime the responder T cells. These T cells may attack the stimulator directly or help B cells to produce MHC-specific antibodies. Hence, whether the organ is rejected by alloantibodies or by cytotoxic T cells or whether the HSC transplanted patient is attacked by the donor T cells (GVHD), the response starts always with the activation of allogeneic T cells.

The strongest alloreaction is directed against incompatible molecules encoded by the Major Histocompatibility Complex (MHC). This can be explained by the fact that these MHC molecules play a fundamental role in antigen presentation and are thus optimally shaped to be recognized by T cells. Furthermore, they are extremely polymorphic so that two individuals virtually always express a different set of MHC molecules.

Three types of immune cells play a essential role in allorecognition:

T cells can recognize a major histocompatibilty molecules in different ways. T cells are also able to recognize allogeneic tissues that are HLA-identical but differ for minor histocompatibility antigens.

Alloreactive T cells can also help B cells to produce alloantibodies that play a crucial role in the rejection of organs.

Natural Killer cells can also recognize allogeneic cells that do not express the same MHC-molecules as the responder (missing self).

But even when the donor and the recipient express the same MHC molecules, the T cell may recognize minor histocompatibilty antigens that are different between donor and recipient.