What are the three major barriers of the human body?

1. Skin-Mucosal Barrier: Skin and mucous membranes are not only mechanical barriers, they also secrete bactericidal substances. So it is important to keep them in an intact state for the sake of health.

2. Blood-brain barrier: the endothelial cell layer of the brain capillaries, which are closely connected, can block pathogenic bacteria in the blood from entering the brain tissue and cerebrospinal fluid, thus protecting the central nervous system.

3. Placental barrier: after the third month of a woman's pregnancy, when an infection occurs, pathogenic bacteria are generally not easy to enter the fetus, and the fetus can be protected from infection.