*7.5.2 Vitamin D (VitD)*

VitD plays an important role for the calcium homeostasis in the bones. Currently, the interest has focused on the modulation of the immune response in fighting viral respiratory infections. Some studies show that patients with deficiencies in VitD are at higher risk of suffering respiratory infections in the upper respiratory tract [79]. One study showed the association of polymorphisms in the VitD receptor and severe bronchiolitis in patients infected by the respiratory syncytial virus [80].

The immunomodulatory effect has been of great importance in some viral infections; several types of cells including epithelial cells treated with VitD induce the expression of the receptor and the production of antimicrobial peptides such as the HBD2. In the patients infected with HIV, the high levels of VitD and its receptor increase the amount of IL-10 and HBD2, which are associated with a natural resistance to HIV infection [62].

The VitD receptor is expressed in several types of cells (monocytes, B cells, T cells and NK), is an endogenous immunomodulator and induces the transcription of the HBD2. This work shows that the expression of HBD2 is important to render tolerance to viral infections [62, 64].
