1. The first line of defense
It 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 acid, gastric acid and enzyme) have bactericidal effect. There are cilia on the respiratory mucosa, which can remove foreign bodies.
2. The second line of defense
It is a bactericidal substance and phagocyte in body fluids.
These two lines of defense are natural defense functions gradually established by human beings in the process of evolution. The characteristic is that people are born with a defense function against a variety of pathogens instead of a specific pathogen, so it is called nonspecific immunity (also known as innate immunity). In most cases, these two lines of defense can prevent pathogens from invading the body.
3. The third line of defense
It is mainly composed of immune organs (thymus, lymph nodes and 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).