Many studies have shown that vitamin A can significantly improve the immune function of chickens. Vitamin A can directly act on B cells, enhance the humoral immune function produced by soluble or granular antigens, participate in and promote the synthesis of antibodies, promote lymphocyte transformation, stimulate the secretion of interleukins and interferons, induce lymphocyte proliferation, and promote Phagocytic cells process the maturation of antigens and helper T cells and enhance the cellular immune function of the body. Certain infectious diseases such as coccidiosis, helminthiasis, etc. can reduce the conversion of vitamin A into vitamin A in the body, resulting in vitamin A deficiency.
In the absence or deficiency of vitamin A, the ability of chickens to synthesize antibodies is diminished, the number of blood lymphocytes is reduced, and the weight of the bursa and thymus is reduced. Dietary supplementation with vitamin A improves the ability of chickens to synthesize antibodies. The diet was supplemented with 4000 IU/kg of vitamin A, and the serum antibody titer was 2 to 5 times that of 1000 IU/kg. Dietary supplementation of 2 μg/g vitamin A, chicken serum IgG, IgM and IgA levels and antibody titers against Newcastle disease virus were significantly higher than without vitamin A (Catherine, 1989). The addition of vitamin A can improve the immune response of chickens to Newcastle disease and Marek vaccine and E. coli, reducing mortality. The chickens were fed with vitamin A deficiency and sufficient diet to infect Newcastle disease virus and E. coli. The results showed that the dietary vitamin A was sufficient, and the ability of chickens to synthesize antibodies and lymphocyte proliferation increased, against Newcastle disease virus and Escherichia coli. Both morbidity and mortality were significantly reduced (Friedman, 1991). Sklan (1994) added 0～13.2μg/g different levels of vitamin A in broiler diets. As the vitamin A level of diet increased, serum antibody titer and T lymphocyte transformation rate increased gradually, at 6.66μg/g. Vitamin A reached its maximum level, and antibody titer and lymphocyte conversion rate began to decrease when it exceeded 6.66 μg/g. It indicates that the level of dietary vitamin A in the highest immunity of the chicken is ten times higher than the recommended amount of 0.52 μg/g (1500 IU/kg) of NRC (1994), but excessive addition of vitamin A will inhibit immunity. Therefore, the level of vitamin A in chicken diets has progressed to a higher level than the addition of nutrient standards, which allows chickens to obtain higher immunity.
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