Vitamin D as a part of the endocrine system is an important component in the interaction between the kidney, bone, parathyroid hormone, and the intestine, which maintains extracellular calcium level within normal limits, in order to keep the vital physiologic process and skeletal integrity. Vitamin D is also associated with hypertension, muscular function, immunity, and ability to encounter infection, autoimmune disease, and cancer. The role of vitamin D in immunity is a feedback reaction of paracrine to eliminate inflammation or to influence CD4 T-cell differentiation and or to increase the function of T suppressor cell or combination between both. The active form of vitamin D produces and maintains self immunologic tolerance, some studies show that 1,25(OH)2D inhibits induction of disease in autoimmune encephalomyelitis, thyroiditis, type-1 diabetes mellitus, inflammatory bowel disease (IBD), and collagen-induced arthritis and Lyme arthritis. when vitamin D levels are normal, dendrites function normally. When vitamin D levels are low, dendrites may not recognize or develop tolerance to normal proteins and cells. This leads to the inappropriate immune response characteristic of autoimmunity. The immune system targets and attacks the bodys own cells and proteins. An autoimmune disease is characterized by abnormal function of the body's immune system that causes the body to attack its own cells and tissues. The most common autoimmune diseases are rheumatoid arthritis, lupus, multiple sclerosis, Type I diabetes and certain thyroid conditions.
How Vitamin D Affects Immunity
Most immune cells contain vitamin D receptors that allow vitamin D to enter cells and exerts its effects on immune cell behavior. In this capacity, vitamin D has been shown to dramatically stimulate the expression of potent antimicrobial peptides. These peptides exist in white blood cells such as neutrophils, monocytes and natural killer cells, and the epithelial cells that line the respiratory tract, where they play a significant role in protecting the lung from infection.
Vitamin D influences both innate and adaptive immunity. The cells of the innate system recognize and respond to pathogens (infectious agents or germs) in a generic way, and the adaptive immune cells have the ability to recognize and remember specific pathogens. They, in turn, generate immunity by mounting stronger attacks each time the same pathogen is encountered. In simpler terms, vitamin D appears to make immune cells better to fulfill their primary function-defense.
Adaptive immunity involves lymphocytes (a specific type of white blood cell) that are able to express a vast number of specific antigen receptors on their cell surface. Should the pathogen be reintroduced at a later point in time, these receptors are activated and the lymphocytes launch an assault against the pathogen. In adaptive immunity, all of the offspring of the activated cells inherit genes, encoding the same receptor specificity. These cells include the memory B cells and memory T cells that are the keys to long-lived specific immunity.
Vitamin D receptors are expressed in monocytes and in activated macrophages, dendritic cells, natural killer cells, and T and B cells. Activation of these receptors by vitamin D have been shown to increase the activity of natural killer cells and enhance the phagocytic activity of macrophages. (Phagocytes are white blood cells that protect the body by, for lack of a better work eating harmful foreign particles, bacteria, etc.) Active vitamin D hormone also increases the production of cathelicidin, an antimicrobial peptide that is produced in macrophages. The release of cathelicidin is triggered by the presence of bacteria, viruses and fungi.
All of these influences enable the immune system to work in a highly efficient manner, reducing the risk of infection and reducing the severity of infections should they strike. These immune pathways are also important in preventing cancer. In fact, higher blood levels of vitamin D are associated with reduced risk of breast, prostate, colon and other cancers. Several intervention studies have shown that vitamin D supplementation is associated with a reduction in cancer incidence of approximately 50 percent and that supplementation of 2,000 IU per day slows the progression of localized prostate cancer in a high percentage of male subjects.
