The second line of defense: bactericidal substances (such as lysozyme) and phagocytes in body fluids.
The third line of defense: through blood circulation and lymphatic circulation, it is mainly composed of immune organs and immune cells.
Moreover, the first line of defense and the second line of defense are also called nonspecific immunity (people are born with defense against many pathogens).
Extended data:
The human body's three lines of defense help the human body resist hundreds of millions of different types of microorganisms, including bacteria, fungi, viruses and parasites. In this global village, we depend on each other, live in peace and safeguard people's health; Once the defense line collapses, various infections will follow.
People often worry about foreign infections, such as the popular COVID-19 (from the animal kingdom). Of course, the invasion of foreign pathogenic microorganisms is indeed a very important threat to human beings, but when the defense line collapses, the endogenous infection is actually the same or even more terrible. For example, there are a large number of various types of microorganisms in our intestines, respiratory tract and oral cavity. Under normal circumstances, it is the existence of these three lines of defense that not only makes them unable to "mess", but makes them "behave".
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