Table 1.

Overview of key roles played by select micronutrients in the immune system [4,9,10,11,12,13,14].

Micronutrient/Role	Innate Immunity	Adaptive Immunity
Vitamin C	Effective antioxidant that protects against ROS and RNS produced when pathogens are killed by immune cells [9,14] Regenerates other important antioxidants such as glutathione and vitamin E to their active state [9] Promotes collagen synthesis, thereby supporting the integrity of epithelial barriers [10] Stimulates production, function and movement of leukocytes (e.g., neutrophils, lymphocytes, phagocytes) [9,14] Increases serum levels of complement proteins [14] Has roles in antimicrobial and NK cell activities and chemotaxis [10] Involved in apoptosis and clearance of spent neutrophils from sites of infection by macrophages [12]	Can increase serum levels of antibodies [12,14] Has roles in lymphocyte differentiation and proliferation [10,12]
Vitamin D	Vitamin D receptor expressed in innate immune cells (e.g., monocytes, macrophages, dendritic cells) [14] Increases the differentiation of monocytes to macrophages [10] Stimulates immune cell proliferation and cytokine production and helps protect against infection caused by pathogens [14] 1,25-dihydroxyvitamin D3, the active form of vitamin D, regulates the antimicrobial proteins cathelicidin and defensin, which can directly kill pathogens, especially bacteria [14]	Mainly inhibitory effect in adaptive immunity [14]; for example, 1,25-dihydroxyvitamin D3 suppresses antibody production by B cells and inhibits T cell proliferation [14]
Vitamin A	Helps maintain structural and functional integrity of mucosal cells in innate barriers (e.g., skin, respiratory tract, etc.) [14] Important for normal functioning of innate immune cells (e.g., NK cells, macrophages, neutrophils) [14]	Necessary for proper functioning of T and B lymphocytes, and thus for generation of antibody responses to antigen [14] Involved in development and differentiation of Th1 and Th2 cells and supports Th2 anti-inflammatory response [10]
Vitamin E	An important fat-soluble antioxidant [10] Protects the integrity of cell membranes from damage caused by free radicals [14] Enhances IL-2 production and NK cell cytotoxic activity [10]	Enhances T cell-mediated functions and lymphocyte proliferation [10] Optimizes and enhances Th1 and suppresses Th2 response [10]
Vitamin B6	Helps regulate inflammation [13] Has roles in cytokine production and NK cell activity [13,15]	Required in the endogenous synthesis and metabolism of amino acids, the building blocks of cytokines and antibodies [14] Has roles in lymphocyte proliferation, differentiation and maturation [14] Maintains Th1 immune response [10] Has roles in antibody production [13]
Vitamin B12	Has roles in NK cell functions [13]	May act as an immunomodulator for cellular immunity, especially with effects on cytotoxic cells (NK cells, CD8+ T-cells) [10] Facilitates production of T lymphocytes [13] Involved in humoral and cellular immunity and one-carbon metabolism (interactions with folate) [13]
Folate	Maintains innate immunity (NK cells) [10]	Has roles in cell-mediated immunity [13] Important for sufficient antibody response to antigens [13] Supports Th 1-mediated immune response [13]
Zinc	Antioxidant effects protect against ROS and RNS [9] Helps modulate cytokine release and induces proliferation of CD8+ T cells [10,16] Helps maintain skin and mucosal membrane integrity [10]	Central role in cellular growth and differentiation of immune cells that have a rapid differentiation and turnover [17] Essential for intracellular binding of tyrosine kinase to T cell receptors, required for T lymphocyte development and activation [9] Supports Th1 response [10]
Iron	Involved in regulation of cytokine production and action [10] Forms highly-toxic hydroxyl radicals, thus involved in the process of killing bacteria by neutrophils [10] Important in the generation of ROS that kill pathogens [14]	Important in the differentiation and proliferation of T lymphocytes [14] Essential for cell differentiation and growth, component of enzymes critical for functioning of immune cells (e.g., ribonucleotide reductase involved in DNA synthesis) [10]
Copper	Free-radical scavenger [4] Antimicrobial properties [14] Accumulates at sites of inflammation, important for IL-2 production and response [13,14] May play a role in the innate immune response to bacterial infections [14]	Has roles in T cell proliferation [13] Has roles in antibody production and cellular immunity [18]
Selenium	Essential for the function of selenium-dependent enzymes (selenoproteins) that can act as redox regulators and cellular antioxidants, potentially counteracting ROS [10,14] Selenoproteins are important for the antioxidant host defense system affecting leukocyte and NK cell function [13]	Involved in T lymphocyte proliferation [4,13] Has roles in the humoral system (e.g., immunoglobulin production) [13]

IL, interleukin; NK, natural killer; RNS, reactive nitrogen species; ROS, reactive oxygen species; Th, helper T cell.