The immune system consists of immune organs, immune cells and immune molecules. lymph nodes, tonsils, appendix, spleen and bone marrow are all important immune organs of the body. Immune cells are produced and developed in the immune organs, and the two most important immune cells in the body are B lymphocytes and T lymphocytes, which eventually differentiate into plasma cells and can produce a variety of antibodies; T lymphocytes can recognize and kill cells infected by pathogens or mutated cells, and can maintain self-tolerance and avoid excessive damage to the body from immune reactions. Immune molecules are synthesized and secreted by immune cells and can participate in the body’s immune response, usually including cytokines such as complement, immunoglobulins, interferons, and interleukins.