The term Vitamin D refers to a group of fat-soluble vitamins. Strictly speaking, vitamin D is not a real vitamin. It's actually a hormone, more specifically a pro-hormone. The body can produce vitamin D itself, but for this it needs the help of the sun. When high-energy rays of the sun hit our skin, the body forms the precursor of the hormone vitamin D with the help of cholesterol.
One form of vitamin D is D3, also known as cholecalciferol or colecalciferol. It is converted into the active hormone calcitriol in the liver and kidneys. In addition, the body can store vitamin D3 in adipose tissue, the so-called storage form of vitamin D, calcifediol (also 25-hydroxy-vitamin-D or 25-OH-vitamin D).
Vitamin D3 and its effect on the immune system
Vitamin D can modulate the innate and adaptive immune response. Vitamin D deficiency is associated with increased autoimmunity and increased susceptibility to infection.
 The immune system defends the body against foreign invading organisms and promotes protective immunity while maintaining tolerance towards itself. The effects of vitamin D deficiency on the immune system have become more evident in recent years, and associated with vitamin D deficiency there appears to be an increased susceptibility to infection in a genetically susceptible host.
Vitamin D has numerous effects on cells within the immune system. It interacts directly with the production of scavenger cells of the immune system, the so-called T-cells and B-cells .
Other effects are the reduced production of messenger substances that promote inflammation and an increased production of messenger substances that contain and inhibit inflammation.
Vitamin D3 and resistance to pathogens
There have been several studies that have linked lower vitamin D levels to increased infection. In a report  almost 19,000 subjects were examined between 1988 and 1994. Individuals with lower vitamin D levels (<30 ng/mL) were more likely to self-report a recent upper respiratory tract infection than those with adequate vitamin D levels.
Another study  men selected from 800 military recruits in Finland according to the vitamin D level in their blood . Those recruits with lower vitamin D levels lost significantly more active duty days due to upper respiratory tract infections than recruits with higher vitamin D levels (above 40 nmol).
Vitamin D deficiency plays a role in susceptibility to infectious diseases. Immune system cells are able to synthesize and respond to vitamin D. It suggests that vitamin D levels (>40ng/ml) have an important impact on the functioning of the immune system. Supplementation in people with vitamin D deficiency can also have a positive effect on the body's defenses.
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 Vitamin D and the Immune System
 Ginde AA, Mansbach JM, Camargo CA., Jr. Association between serum 25-hydroxyvitamin D level and upper respiratory tract infection in the Third National Health and Nutrition Examination Survey. Arch Intern Med. 2009;169(4):384–90. [PubMed]
 Laaksi I, et al. An association of serum vitamin D concentrations < 40 nmol/L with acute respiratory tract infection in young Finnish men. Am J Clin Nutr. 2007;86(3):714–7. [PubMed]
 Chen S, et al. Modulatory effects of 1,25-dihydroxyvitamin D3 on human B cell differentiation. J Immunol. 2007;179(3):1634–47. [PubMed]