Tolerance to own tissue and rejection of transplanted organs
Typically, the immune system can differentiate between self-cells and non-self-cells (particularly germs and viruses) in the human body. Although scientists have been investigating how the immune system differentiates self-cells from non-self-cells, the immune’s unresponsiveness to its cells is referred to as self-tolerance or tolerance to its tissue. The concept of self-tolerance can be discussed through consideration of the lymphocytes—the B cells and T cells. The lymphocytes’ ability to ignore self-cells can be explained through the concept of central tolerance and peripheral tolerance (see the diagram below).
Figure 1: Central and peripheral tolerance. Source: (Gregersen & Behrens, 2006)
Central tolerance, which is scientifically referred to as the negative selection, is the process through which lymphocytes with receptors that are responsive to self-cells, are eliminated during the early stages of lymphocyte development. In other words, when the lymphocytes are developing in the primary lymphoid—such as thymus and bone marrow—developing T cells and B cells that are reactive to self-cells undergo apoptosis or undergo a modification of the antigen receptors. On the other hand, lymphocytes that receive the correct antigen signals are released from the primary lymphoid organs into the periphery. However, central tolerance is not faultless. In some cases, self-reactive cells can escape into the periphery and attack self-cells. Due to this reason, there is a secondary mechanism—peripheral tolerance—that guarantees that lymphocytes do not attack self-cells once they leave the thymus and the bone marrows to the periphery. In this regard, peripheral tolerance acts as an intermediary between the lymphoid organs and the periphery; hence, it takes place once developing T-cells and B-cells exit the primary lymphoid organs. Peripheral tolerance eliminates self-responsive cells through different mechanisms that include; deletion of auto-reactive T cells or programmed cell death, induction of energy, and development of induced regulatory T cells.
On the other hand, the rejection of transplants organs happens when the transplanted organ is rejected and attacked by the T-lymphocytes. Usually, the T-cells identified the transplanted organs as foreign material in the body, thereby becoming reactive to the organ.
References
Gregersen, P. K., & Behrens, T. W. (2006). Genetics of autoimmune diseases—disorders of immune homeostasis. Nature Reviews Genetics, 7(12), 917-928.