Vitamin D is suggested to be an important immune system regulator. 1,25 dihydroxyvitamin D (1,25(OH)2D3), which is the active form of vitamin D, decreases the proliferation of purified T-helper (Th)1 cells as well as the production of interferon (IFN) γ, interleukin (IL)-2, IL-5 and tumor necrosis factor-alpha (TNF-)α. In Th2 cells, 1,25 (OH)2D3 stimulates IL-4 and transforming growth factor TGF production, which in turn may suppress inflammatory T cell activity. In the absence of vitamin D signaling, the T cell compartment has a potentially stronger Th1 phenotype. Furthermore, 1,25(OH)2D3 inhibits dendritic cell (DC) differentiation and maturation, leading to down-regulated expression of major histocompatibility complex class II molecules (MHC-II), co-stimulatory molecules and IL-12; enhances IL-10 production and promotes DC apoptosis. Because of these effects, 1,25(OH)2D3 inhibits DC-dependent T cell activation. In vitro, it is determined that 1,25(OH)2D3 stimulates phagocytosis and killing of bacteria by macrophages, but suppresses the antigen presenting capacity of these cells and dendritic cells. Additionally, Chen et al. have suggested that vitamin D might have a role in regulating antibody production. They have found that 1,25(OH)2D3 not only inhibits activated B cell proliferation but induces their apoptosis as well.