Regulation of Immune Function by Vitamin D and Its Use in Diseases of Immunity An-Sofie Vanherwegen, MSc, Conny Gysemans, PhD*, Chantal Mathieu, MD, PhD INTRODUCTION The immune system, innate and adaptive, is a highly evolved and complex network, essential for promoting survival in an environment full of potential pathogens, while maintaining self-tolerance. It is now well-established that vitamin D 3 plays a prominent role in immune health on top of its classical effects on calcium and bone homeostasis. The VDR is expressed in all immune cells, and several of these cells are also capable of synthesizing and/or responding to the bioactive metabolite, allowing for autocrine and paracrine actions of vitamin D. 1 It is not surprising that vitamin D insufficiency and defi- ciency seem to increase susceptibility to bacterial and viral infections and, in geneti- cally susceptible individuals, to autoimmunity. Vitamin D insufficiency and deficiency Disclosure Statement: The authors have nothing to disclose. Laboratory of Clinical and Experimental Endocrinology (CEE), KU Leuven, O&N1 Herestraat 49 - bus 902, Leuven 3000, Belgium * Corresponding author. E-mail address: conny.gysemans@kuleuven.be KEYWORDS  Vitamin D 3  Macrophage  Dendritic cells  Neutrophils  B cell T cells  Autoimmunity  Infections KEY POINTS  Vitamin D deficiency, owing to low dietary intake or low sunlight exposure, is an environ- mental risk factor for autoimmune and infectious diseases.  The bioactive form of vitamin D has a wide variety of immunomodulatory effects in innate and adaptive immune cells.  Vitamin D enhances the antimicrobial activity in macrophages relevant for defense against infectious diseases.  The role of vitamin D supplementation in a situation of vitamin D sufficiency remains unclear.  Beneficial effects on disease risk and severity in animal studies and some human trials has been shown. Endocrinol Metab Clin N Am 46 (2017) 1061–1094 http://dx.doi.org/10.1016/j.ecl.2017.07.010 endo.theclinics.com 0889-8529/17/ª 2017 Elsevier Inc. All rights reserved.