3.3.2.1. Central T lymphocyte tolerance

The immature T cells die by apoptosis, whenever encounter with self-high avidity protein antigens in the thymus. The immature lymphocytes in the thymus can recognize both self and non-self-antigens. If a self-antigen high in concentration and avidity meet with immature lymphocyte, lymphocytes receives signals that trigger apoptosis, finally dies. This is known as negative selection. Since the self-protein antigens are expressed mainly in thymus because of transcription factor responsible called AIRE (for autoimmune regulator), they are high in concentration [4, 19].

Some lymphocytes which escape from negative selection, mature to dangerous self-reactive T cells with CD4+ T and CD8+ T. They recognize self-antigens through class I and II MHC molecules [4, 18, 19].

And some other develop into regulatory T cells which regulate mostly suppress both naïve and memory T cell responses by a cell to cell contact and by down-regulating the expression of cytokines and co-stimulatory molecules on the antigen-presenting cells. Unfortunately this is not antigen specific reaction [4, 5, 18, 19].

### 3.3.2.2. Peripheral T lymphocytes tolerance


3. Immune suppression: Autoreactive mature T lymphocytes that encounter with self-antigen may develop into regulatory cells which suppress the self-reactive lymphocytes response [4, 18, 19].
