Fig. 4

The biology of Th cells at the periphery. a Schematic illustration of the activation of Th cells upon antigen stimulation at the periphery. According to “the second touch” hypothesis, full Th polarization needs a second antigen presentation to primed Th cells by APCs in the inflamed tissues. b Schematic summary of peripheral tolerance mechanisms against autoreactive Th cells, which escape from thymic negative selection. Ag, antigen; AICD, activation-induced cell death; APC, antigen-presenting cell; DC, dendritic cell; pTh, primed Th cell