A fat-soluble vitamin that cannot be synthesized by mammals and other vertebrates and must be provided in the diet. Green plants contain precursors of the vitamin, notably carotenes, that are converted to vitamin A in the intestinal wall and liver. It is also present in dairy products. The aldehyde derivative of vitamin A, retinal, is a constituent of the visual pigment rhodopsin. Deficiency affects the eyes, causing night blindness, xerophthalmia (dryness and thickening of the cornea), and eventually total blindness. Vitamin A is also important for cell differentiation and growth, especially in maintaining the integrity of epidermal and mucosal surfaces, which act as a barrier against infection. The competence of macrophages, which are crucial to innate immunity, requires vitamin A, as does the proper utilization of stored iron in the manufacture of red blood cells (haemopoiesis); vitamin A deficiency can result in iron deficiency anaemia and defective immunity.