The human body has an immune system, which mainly consists of immune organs and immune cells. The immune organs are mainly composed of bone marrow, thymus, spleen, lymph nodes, etc.; the immune cells are mainly composed of macrophages, neutrophils, lymphocytes, etc. Bone marrow is the central blood-forming organ and the birthplace of various immune cells. 1, the function of the immune system (1) immune defense Even if you take a bath every day, there are at least 10 billion bacteria settled in the eyelashes, nostrils, mouth, armpits, navel, groin, skin and other parts, not to mention countless fungi and parasites (some data show). The environment around people is not clean, in the air is not circulated in the room, the bacteria per 1 cubic meter is up to 15,000. So many pathogenic microorganisms in people and the surrounding environment, but people can still live a healthy life, thanks to the immune defense function of the human immune system, which can resist the human invasion of pathogenic microorganisms and avoid the occurrence of infectious diseases. (2) Immune self-stabilization Human cells are metabolizing every moment, and a large number of aging and damaged cells are produced every day. The immune system is able to identify these cells and remove them in a timely manner, thus maintaining the physiological stability of the human body. (3) Immunosurveillance Australian immunologist and Nobel laureate Burnet proposed the theory of immunosurveillance in 1967: the body’s immune system identifies and kills mutated self-cells through cellular immune mechanism, so that the mutated cells can be removed before they come to form tumors. If the immune surveillance function is low, tumors can occur! The genetic loci of normal cells are subject to errors during each replication, and therefore some mutant cells often appear. It is estimated that in a healthy person, 300 to 400 mutated cells can be produced every day, and as we age, the number of mutated cells will increase to more than 3000 per day, which is the intrinsic basis for cancer cell formation. Although many normal cells in the body mutate every day, only a small number of cells develop into cancer cells. Because the immune system can play a surveillance role to identify these mutated cells and remove them in time. 2.Relationship between immunity and cancer (1) Low immunity is prone to cancer Scientists transplanted cancer cells into experimental mice, and if the mice have normal immunity, cancer is less likely to occur; if the mice’s thymus gland is removed, or immunosuppressive drugs are injected to reduce the immunity of the mice, the cancer cells are easily inoculated successfully. The experimental results confirmed that it is the occurrence of cancer after immune deficiency first, and some immunosuppressive drugs can induce cancer, for example, one case of kidney failure patient who took immunosuppressive drugs for a long time to resist rejection after kidney transplantation, and liver cancer occurred several years later. (2) Cancer is prone to development and metastasis due to low immunity. Immune cells in human body can effectively kill cancer cells, and natural killer cells (NK cells) are the sentinels of immune system. NK cells are the first line of defense against cancer cells. When NK cells come into contact with cancer cells and combine with them, they release “perforating cords” that form holes in the cancer cell membranes, and then inject toxic “granzyme” into the cancer cells, causing them to die. Macrophages are another important type of anti-cancer cells. Activated macrophages fuse with cancer cells and kill them by releasing lysosomal enzymes and also by secreting cytotoxic substances such as reactive oxygen species, reactive nitrogen species and protein hydrolases. When immunity is low, the fighting power of these immune cells will be reduced and cancer cells will easily grow and spread. Pathological biopsy shows that if there are obvious macrophages around the cancer mass, the incidence of cancer spreading and metastasis will be lower and the survival period will be longer; on the contrary, the incidence of cancer spreading and metastasis will be high and the patient’s survival period will be shorter. (3) Cancer reduces immunity Cancer cells do not passively wait for the attack of immune cells, but use some means in order to weaken the fighting power of immune cells. The specific methods include: inducing the production of suppressive cells; inducing the secretion of suppressive cytokines; and secreting some products with immunosuppressive effect by themselves. So early stage often shows tumor local immune deficiency, and late stage often is systemic immune deficiency, metastatic cancer can also use some means to reduce the immune cell killing power. Researchers have demonstrated that after removing metastatic cancer masses during surgery, isolating the lymphocytes from them and placing them individually in a glass dish for a period of time, it was found that these lymphocytes regained their fighting power and were able to kill cancer cells. However, when they were in close proximity to metastatic cancer cells in the body, their killing power was inhibited again, that is, the cancer cells formed metastatic lesions that inhibited or resisted the killing power of neighboring lymphocytes. The relationship between immunity and cancer can be summarized as follows: Low immunity? Cancer development and progression? Further decline of immunity? More and more cancer cells, less and less immunity. When cancer cells reach 1 trillion, the patient faces death.