There are several reasons for tumor formation as follows: 1. Nutritional factors: animals lacking vitamin A are susceptible to tumors induced by chemical carcinogenic substances, while administration of vitamin A or vitamin A acid can reduce the occurrence of tumors. The mechanism of vitamin A inhibiting tumorigenesis may be as follows: (1) Carcinogenic substances need to be metabolized before they have carcinogenic activity. Vitamin A and its analogues can inhibit the activity of microsomal oxidase in liver and lung, thus reducing the carcinogenic activity in the body. (2) Vitamin A plays an important role in maintaining the differentiation of epithelial cells into special tissues, and the generation of tumor is the result of the damage of differentiation. (2) Immune factors: The cellular immune status (such as lymphocyte transformation rate) of many tumor patients is lower than that of normal population. The incidence of malignant tumor is higher than that of the normal population in people with congenital immunodeficiency or long-term application of immunosuppressive drugs. The immunosurveillance hypothesis suggests that the immune system of the normal organism has inhibitory and rejection effects on a few cancerous cells or cancer cells. If the immune status of the body is inhibited or destroyed, some antibodies or factors produced by the body close the cancer cells so that the cancer cells are not subject to immune surveillance, which can cause tumor. Mental factors: personality, emotion and mental tension are related to tumor.