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Immune Function
Vitamin A plays a role in many areas of the immune system, particularly in T cell differentiation and proliferation.

Vitamin A promotes the proliferation of T cells through an indirect mechanism involving an increase in IL-2. In addition to promoting proliferation, Vitamin A, specifically retinoic acid, influences the differentiation of T cells. In the presence of retinoic acid, dendritic cells located in the gut are able to mediate the differentiation of T cells into regulatory T cells. Regulatory T cells are important for prevention of an immune response against "self" and regulating the strength of the immune response in order to prevent host damage. Together with TGF-β, Vitamin A promotes the conversion of T cells to regulatory T cells. Without Vitamin A, TGF-β stimulates differentiation into T cells that could create an autoimmune response.

Hematopoietic stem cells are important for the production of all blood cells, including immune cells, and are able to replenish these cells throughout the life of an individual. Dormant hematopoietic stem cells are able to self-renew and are available to differentiate and produce new blood cells when they are needed. In addition to T cells, Vitamin A is important for the correct regulation of hematopoietic stem cell dormancy. When cells are treated with all-trans retinoic acid, they are unable to leave the dormant state and become active, however, when vitamin A is removed from the diet, hematopoietic stem cells are no longer able to become dormant and the population of hematopoietic stem cells decreases. This shows an importance in creating a balanced amount of vitamin A within the environment to allow these stem cells to transition between a dormant and activated state, in order to maintain a healthy immune system.

Vitamin A has also been shown to be important for T cell homing to the intestine, effects dendritic cells, and can play a role in increased IgA secretion which is important for the immune response in mucosal tissues.

Article: T reg cells
Regulatory T cells are important for anti-inflammatory responses. Vitamin A and TGF-beta promote T cell differentiation into regulatory T cells opposed to Th17 cells, even in the presence of IL-6.

The small intestinal environment is high in vitamin A and is a location where retinoic acid is produced. The retinoic acid and TGF-beta produced by dendritic cells within this area signal for production of regulatory T cells. The intestinal environment can lead to induced regulatory T cells (iTregs) with TGF-beta and retinoic acid. The Tregs within the gut are differentiated from naïve T cells after antigen is introduced.