What is "immune tolerance"

Tolerance \Tol"er*ance\, n. [L. tolerantia: cf. F. tol['e]rance.]

1. The power or capacity of enduring; the act of enduring; endurance.

   Diogenes, one frosty morning, came into the market place, shaking, to show his tolerance.
   --Bacon.

2. The endurance of the presence or actions of objectionable persons, or of the expression of offensive opinions; toleration.

3. (Med.) The power possessed or acquired by some persons of bearing doses of medicine which in ordinary cases would prove injurious or fatal.

4. (Forestry) Capability of growth in more or less shade.

5. the allowed amount of variation from the standard or from exact conformity to the specified dimensions, weight, hardness, voltage etc., in various mechanical or electrical devices or operations; -- caklled also allowance specif.: (Coinage) The amount which coins, either singly or in lots, are legally allowed to vary above or below the standard of weight or fineness.

6. (Biochemistry) the capacity to resist the deleterious action of a chemical agent normally harmful to the organism; as, the acquired tolerance of bacteria to anitbiotics.

7. (Immunology) the acquired inability to respond with an immune reaction to an antigen to which the organism normally responds; -- called also immunotolerance, immunological tolerance, or immune tolerance. Such tolerance may be induced by exposing an animal to the antigen at a very early stage of life, prior to maturation of the immune system, or, in adults, by exposing the animal to repeated low doses of a weak protein antigen ( low-zone tolerance), or to a large amount of an antigen ( high-zone tolerance).

   Tolerance of the mint. (Coinage) Same as Remedy of the mint. See under Remedy.

Immune tolerance, or immunological tolerance, or immunotolerance, describes a state of unresponsiveness of the immune system to substances or tissue that have the capacity to elicit an immune response. It contrasts with conventional immune-mediated elimination of foreign antigens (see Immune response). Tolerance is classified into central tolerance or peripheral tolerance depending on where the state is originally induced—in the thymus and bone marrow (central) or in other tissues and lymph nodes (peripheral). The mechanisms by which these forms of tolerance are established are distinct, but the resulting effect is similar.

Immune tolerance is important for normal physiology. Central tolerance is the main way the immune system learns to discriminate self from non-self. Peripheral tolerance is key to preventing over-reactivity of the immune system to various environmental entities ( allergens, gut microbes, etc.). Deficits in central or peripheral tolerance also cause autoimmune disease, resulting in syndromes such as systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, autoimmune polyendocrine syndrome type 1 (APS-1), and immunodysregulation polyendocrinopathy enteropathy X-linked syndrome (IPEX), and potentially contribute to asthma, allergy, and inflammatory bowel disease. And immune tolerance in pregnancy is what allows a mother animal to gestate a genetically distinct offspring with an alloimmune response muted enough to prevent miscarriage.

Tolerance, however, also has its negative tradeoffs. It allows for some pathogenic microbes to successfully infect a host and avoid elimination. In addition, inducing peripheral tolerance in the local microenvironment is a common survival strategy for a number of tumors that prevents their elimination by the host immune system.