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Vitamin D and Immunity: A comprehensive review of its impact on autoimmunity, allergy suppression, antimicrobial defense, and cancer inhibition. The Egyptian journal of immunology Vitamin D, commonly known for its impact on bone metabolism, is vital in various bodily processes, including regulating immune responses. The actions of vitamin D are carried out through its receptor, found in cells of different human organs and tissues, particularly in most immune system cells and epithelial cells. After binding to the receptor, vitamin D forms a complex with vitamin A and its receptor in the cytoplasm. This complex can inhibit or enhance the transcription of hundreds of genes, including those that control cell growth, differentiation, apoptosis, and prevent malignant growth and angiogenesis. Studies have shown that vitamin D weakens antigen presentation by dendritic cells, shifts the balance of Th1/Th2 cell responses towards Th2, and promotes the development and activity of Treg cells. Additionally, vitamin D enhances the production of "endogenous antibiotics" against bacteria, fungi, and viruses. This important nutrient has been linked to preventing autoimmune and atopic diseases, respiratory infections, and tumors. A lack of vitamin D, or hypovitaminosis D, is present in almost half of the population and is a leading cause of weakened immunity and increased morbidity. Thus, detecting, preventing, and treating hypovitaminosis D should be a priority in healthcare in the Kingdom of Saudi Arabia.