The immune system consists of three major components: immune organs, immune cells, and immune molecules. The immune organs are divided into central and peripheral immune organs, which are differentiated according to their early differentiation and different functions. The first line of protection is usually composed of immune tissues, which resist the invasion of germs and also have a bactericidal effect. The second line of protection is made up of immune cells that produce humoral and cellular immunity and can form antibodies that can stop pathogens from attacking the body. The third line of defense is composed of immune organs that mature as the body grows and develops. The entire immune system plays a vital role in the body’s prevention of bacterial and viral invasion. The immune system is widely distributed throughout the body and also plays a major role in the mucosal anti-infective immune action.