The first line of defense of human body is composed of skin and mucous membrane, which can not only prevent pathogens from invading human body, but also its secretions (such as lactic acid, fatty acids, gastric acid, enzymes, etc.) have bactericidal effect. The second line of defense is the bactericidal substances and phagocytes in body fluids. These two lines of defense are natural defense functions gradually established by human beings in the process of evolution. Their characteristics are innate, and they do not target a specific pathogen, but have defensive effects on many pathogens, so they are called nonspecific immunity (also known as innate immunity). The third line of defense mainly consists of immune organs (thymus, lymph nodes, bone marrow, spleen, etc.). ) and immune cells (lymphocytes). The third line of defense is the acquired defense function gradually established by the human body after birth, which is characterized by being born and only acting on specific pathogens or foreign bodies, so it is called specific immunity (also known as acquired immunity).
] briefly summarize 1. Skin and mucous membrane II. Bactericidal substances and phagocytes in body fluids. Antibodies produced by immune organs and immune cells.