ABSTRACT
DCs play a pivotal role in the immune response by providing signals that direct naïve T helper (Th) cells to proliferate and differentiate into Th1, Th2, or T regulatory cells. Three signals are delivered by an APC that are thought to determine the fate of naïve T cells (Figure 24.1) [4]. Recently it has been shown that the signals delivered by the APC to the T cell determine its polarization into a specifi c type of effector cell, referred to as signal 3 (Figure 24.1). T helper 1 cells activate cytotoxic T cells and stimulate the microbicidal properties of macrophages (cell-mediated immune response). T helper 2 cells initiate the humoral immune response by activating naïve antigen-specifi c B cells to produce antibodies. The polarizing signals are mediated by various soluble and membrane-bound factors, such as interleukin-12 (IL-12) or CC-chemokine ligand 2 (CCL2) [5].
