Positive selection: Thymic cortex. T cells expressing TCRs capable of binding self-MHC on cortical epithelial cells survive.

Negative selection: Thymic medulla. T cells expressing TCRs with high affinity for self antigens undergo apoptosis or become regulatory T cells. Tissue-restricted self-antigens are expressed in the thymus due to the action of autoimmune regulator (AIRE); deficiency leads to autoimmune polyendocrine syndrome-1.

Lymph node:Helper T cell → IL-12, IFN-γ → Th1Helper T cell → IL-2, IL-4 → Th2Helper T cell → TGF-β, IL-6 → Th17Heper T cell → TGF-β → Treg