The body’s three lines of immune defense are the skin-mucosal barrier, germicidal substances in body fluids and phagocytes, etc., and adaptive immunity. 1. Skin and mucous membrane barrier: The first line of defense is the skin and mucous membrane barrier, which includes hair, skin, mucus, and bactericidal substances secreted by the mucous membrane on the surface of the skin and mucous membrane. The normal flora on the skin and mucosal surface, etc., can resist the invasion of foreign substances from the outside into the body. 2. Bactericidal substances and phagocytes in body fluids: Bactericidal substances and phagocytes in body fluids belong to the second line of immune defense, with the role of dissolving, phagocytosis and elimination of pathogens, and this immune ability is available to people at birth. 3. Adaptive immunity: also known as specific immunity, constitutes the body’s third line of immune defense, refers to the body to accept antigenic stimulation, the body antigen-specific lymphocyte clonal recognition of antigens, activation, monoclonal proliferation, differentiation, or incapacitation, apoptosis, and then play a biological effect of the whole process. It is recommended to protect the immune system by regularizing work and rest and increasing exercise in daily life. If there is any immune abnormality, it is recommended to go to the hospital in time.