Tolerance (immunology)
Overview
Tolerance in immunology is the process by which the immune system does not attack an antigen. It is a state of unresponsiveness of the immune system to substances or tissue that have the capacity to elicit an immune response in a given organism. It is achieved through several mechanisms including clonal deletion, clonal anergy, and immune suppression.
Mechanisms of Tolerance
There are two types of tolerance in the immune system, namely central tolerance and peripheral tolerance.
Central Tolerance
Central tolerance is the process of eliminating any developing T or B lymphocytes that are reactive to self. This process occurs in the thymus for T cells and in the bone marrow for B cells, hence the term "central". Central tolerance is primarily achieved through clonal deletion, which involves apoptosis of autoreactive cells.
Peripheral Tolerance
Peripheral tolerance is the second form of immunological tolerance. It occurs in the periphery after lymphocytes have matured and left the central lymphoid organs. Mechanisms of peripheral tolerance include clonal anergy, immune suppression, and clonal ignorance.
Role in Autoimmunity
Tolerance is crucial in preventing autoimmune diseases, which occur when the immune system mistakenly attacks self tissues. Breakdown of tolerance leads to autoimmunity. There are several factors that contribute to the breakdown of tolerance including genetic factors, environmental triggers, and stochastic events.
Clinical Significance
Understanding the mechanisms of tolerance is important in several clinical scenarios. For example, in organ transplantation, tolerance is desired to prevent rejection of the transplanted organ. In cancer, tolerance is detrimental as it allows the immune system to ignore tumor cells.
See Also
References
1. Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 8th edition. Philadelphia: Elsevier; 2014. 2. Janeway CA Jr, Travers P, Walport M, et al. Immunobiology: The Immune System in Health and Disease. 5th edition. New York: Garland Science; 2001. 3. Murphy K, Travers P, Walport M. Janeway's Immunobiology. 8th edition. New York: Garland Science; 2012.