ABSTRACT
This chapter focuses on the role of vitamin A in the various parts of the immune system, not only as an important factor in maintaining the integrity of mucosal epithelial barriers, but also its role in regulating the functions of the different immune cells. It highlights the molecular mechanisms whereby the vitamin A metabolites exert their immune modulatory effects. Innate immune defense includes circulating phagocytes and innate lymphoid cells (ILCs), including conventional natural killer (NK) cells, as well as complement proteins, acute-phase proteins, and pattern-recognizing receptors. ILCs were initially linked to development of lymphoid tissues, but have later been given roles in inflammation at barrier surfaces in response to infection and tissue damages, and to participate in the transition from innate to adaptive immunity. It should be emphasized that regulation of catabolism of all-trans retinoic acid is an equally important mechanism for controlling the levels of retinoic acid in various cells and tissues.
