Figure 2.Role of APCs, Th1 and Th2 cells, and type 1/pro-inflammatory and type 2/anti-inflammatory cytokines in the regulation of cellular and humoral immunity. Cellular immunity provides protection against intracellular bacteria, protozoa, fungi and several viruses, while humoral immunity provides protection against multicellular parasites, extracellular bacteria, some viruses, soluble toxins and allergens (see text). The cellular source(s) of IL-4 that directs the differentiation of Th0 cells towards the Th2 phenotype is not well defined. Solid lines represent stimulation, while dashed lines inhibition.Abbreviations: Ag, antigen; APC, antigen-presenting cell; DC, dendritic cell; IFN, interferon; IL, interleukin; Mo, monocyte; NK, natural killer cell; T, T cell (lymphocyte); Th, T helper cell (lymphocyte); TNF, tumor necrosis factor.