[Vitamin D as an immuno-hematopoietic hormone. New perspectives on a previously known substance]

1,25-dihydroxyvitamin D3 (1.25[OH]2D3), the most active metabolite of vitamin D3, plays an important role in bone and mineral metabolism. There is growing evidence that the vitamin D endocrine system may be important in other systems as well: 1,25(OH)2D3 is synthesized in extrarenal tissue. Macrophages from both normal human individuals and patients with sarcoidosis are capable of producing 1,25(OH)2D3. Specific receptors for 1,25(OH)2D3 are present in a number of cells and tissues other than those of the classical target organs (bone, intestine and kidney). The possible role of 1,25(OH)2D3 in both the modulation of immune responses and normal and leukemic hematopoiesis is discussed.