The immune system is composed of immune cells, immune tissues and immune organs, which, in layman’s terms, form three lines of protection in the human body. The first line of defense is usually composed of immune tissues, including skin and mucous membranes, which can resist the invasion of pathogens and have a bactericidal effect. The second line of defense is often composed of immune cells, including B cells and T cells, which can produce humoral and cellular immunity to antigen-antibody specific binding, as a way to prevent pathogens from attacking the body. The third line of defense is often composed of immune organs, including the thymus, the central immune organ, and the spleen, the peripheral immune organ, which are characteristically created after birth, as these organs mature as the body grows and develops. These three lines of defense are also interconnected and help each other, all inextricably linked, and it is sufficient to be able to understand this mechanism simply.