The human immune system has three major functions: immune surveillance, immune response and immune memory, which are described as follows: 1. Immune surveillance: the body can recognize pathogens such as bacteria and viruses through macrophages, neutrophils, natural killer cells, and innate immune cells, and then pass this information to lymphocytes; 2. Immune response: also known as the immune response, this stage is mainly participated by lymphocytes involved, including antigen recognition, cell activation and immune killing. Through antigen recognition and cell activation, the immune system produces many different types of effector substances, such as antibodies, killer T cells, T effector cells, etc. Antibodies can combine coagulation and modulation to activate complement and antibody-dependent cell-mediated toxicity, thereby eliminating the foreigner. contact with antigens by T effector cells can induce immune inflammation, followed by direct killing of target cells by killer T cells. The human body, through innate immune response, can perform extensive clearance of foreign invading pathogens, or mutated cells in the body, and implement precise strikes through acquired immunity; 3. Immune memory: It allows the body to rapidly initiate secondary immunity after being exposed to the same antigenic stimulus again, and to exert a stronger immune response, so that the pathogen is completely cleared, or is in a temporary state of repression.