THE EFFECT OF CIRCULATING VITAMIN D LEVELS ON PHAGOCYTIC FUNCTION AMONG HEALTHY ADULT EGYPTIANS

Abstract

Extracellular calcium is vital for the function of many metabolic processes and neuromuscular activities. Vitamin D plays a primary physiological role in maintaining extracellular calcium ion levels in the human body. Vitamin D influences calcium levels primarily by controlling the absorption of calcium from the intestine, through direct effects on bone and also through its effects on PTH secretion (Holick, 2002).

However, vitamin D also has a physiological role beyond its well-known role in skeletal homeostasis (Bikle, 2009).Vitamin D has an important role in both innate and adaptive immune responses as receptors for vitamin D are present in various immune cells, including monocytes, macrophages and dendritic cells, as well as T and B lymphocytes (Veldman et al., 2000). Besides being targets, immune cells express vitamin D-activating enzymes allowing local conversion of inactive vitamin D into active vitamin D within the immune system (Bell, 1998).

Vitamin D has two forms : the inactive form 25(OH) D which is the main circulating form that undergoes hydroxylation in the kidney under the influence of 1α-hydroxylase enzyme to yield the biologically active form 1, 25(OH)2 D. While 1, 25(OH)2 D is the biologically active “hormonal” form of the vitamin, many health effects correlate better with circulating 25(OH) D levels. This is due to extrarenal presence of the enzyme 1α-hydroxylase, which leads to intracellular formation of 1, 25(OH)2 D in many target tissues, resulting in high local concentrations. Therefore, many studies focus only on 25(OH) D levels which reflect the exact level of the vitamin (Jones et al., 2007).