Peripheral tolerance is immunological tolerance

Immune tolerance

Immune tolerance or immunological tolerance is the process by which the immune system does not attack an antigen. It can be either 'natural' or 'self tolerance', in which the body does not mount an immune response to self antigens, or 'induced tolerance', in which tolerance to external antigens can...

 developed after T and B cells mature and enter the periphery. These include the suppression of autoreactive cells by 'regulatory' T cells and the generation of hyporesponsiveness (anergy) in lymphocytes which encounter antigen in the absence of the co-stimulatory signals that accompany inflammation, or in the presence of co-inhibitory signals.

Ignorance

Potentially self-reactive T-cells are not activated at immunoprivileged sites, where antigens are expressed in non-surveillanced areas. This can occur in the testes, for instance. Anatomical barriers can separate the lymphocytes from the antigen, an example is the central nervous system (the blood-brain-barrier). Naive T-cells are not present in high numbers in peripheral tissue, but stay mainly in the circulation and lymphoid tissue.

Some antigens are at too low a concentration to cause an immune response - a subthreshold stimulation will lead to apoptosis in a T cell.

Immunological privileged areas

Includes the brain, the anterior chamber of the eye, the testis and the foetus. These areas are protected by several mechanisms:
Fas-ligand expression bind Fas on lymphocytes, which dies in apoptosis, anti-inflammatoric cytokines (including TGF-beta and IL-10) and blood-tissue-barrier with tight-junctions between the endothelial cells.

In the placenta IDO

Indoleamine 2,3-dioxygenase

Indoleamine-pyrrole 2,3-dioxygenase is an immunomodulatory enzyme produced by some alternatively activated macrophages and other immunoregulatory cells...

(indolamine 2,3 dioxygenase) breaks down tryptophan, creating a "tryptophan desert" micro environment which inhibits lymphocyte proliferation.

Split Tolerance

As many pathways of immunity are interdependent, they do not all need to be tolerised. For example, tolerised T cells will not activate autoreactive B cells. Without this CD4+ T-cell help ,the B cells will not be activated.

Induced anergy

T-cells can be made non-responsive to antigens presented if the T-cell engages an MHC molecule without co-stimulatory molecules. This will occur if there is no acute inflammation, leading to no co-stimulator upregulation due to the low concentration of cytokines.

Suppression

Auto-reactive T-cells are prevented from reacting due to the presence of T-reg cells. Experimentally, removal of T-reg cells leads to autoimmune reactions.