Figure 1: Cellular and humoral immune responses in HCV infection. Plasmacytoid dendritic cells (pDC) recognize HCV infection and produce IFN-α, which activates natural killer (NK) cells, helper T (Th) cells, macrophages, and cytotoxic T lymphocytes (CTLs). Activated NK cells destroy the HCV-infected hepatocytes in a nonspecific manner, whereas CTLs destroy the infected hepatocytes in an antigen-specific manner. Myeloid dendritic cells (mDC), which recognize dead hepatocytes, secrete IL-12, promoting the activation of NK cells, Th1 cells, and CTLs. Activated Th1 cells, in turn, promote DC maturation by interacting with the CD40/CD40 ligand. Macrophages stimulated by type 1 helper T (Th1) cells produce TNF-, which accelerates local inflammation. In humoral immune responses, Th2 cells activate B cells. Plasma cells differentiated from B-cells secrete immunoglobulins to neutralize the circulating HCV. Abbreviated terms: CTL, cytotoxic T lymphocyte; pDC, plasmacytoid dendritic cells; mDC, myeloid dendritic cells; Th1 cell, type 1 helper T cell; Th2 cell, type 2 helper T cell; NK cell, natural killer cell; IFN, interferon; IL, interleukin; TNF, tumor necrosis factor.