Figure 8.

Crucial biological and physiological functions of calcitriol [1,25(OH)2D] in various body systems, focusing on the immune system. Intracellularly generated calcitriol stimulates both innate and adaptive immune systems. Essential immunological functions of vitamin D, such as anti-inflammatory, anti-microbial, and antioxidant activities, enables overcoming invading microbes like bacteria and viruses (depicted in purple ovals). Vitamin D sufficiency positively modulates immune cells (depicted in red ovals), reduces multiplication, increases the destruction of pathogens like SARS-CoV-2, and tightens gap junctions, thus preventing fluid extravasation and the spread of microbes. Not included are, increased expression of ACE-2 and reduced concentration of angiotensin-II, which diminishes the risk of cytokine storm, acute respiratory distress syndrome (ARDS), and death (modified from Wimalawansa, S.J., 2020, [16]).