How does the human body defend against pathogens?

The human body has established three lines of defense. The first line of defense is contact with the outside world, such as skin, respiratory tract, gastrointestinal tract and urinary tract. Skin is the first line of defense in contact with the outside world. When we play in the sand pit or swim in the water, the germs in the environment are kept out because of the protection of intact skin. In addition, we often say that the disease enters from the mouth, because the gastrointestinal tract is also an important line of defense for contact with the outside world. If rotten or unclean food accidentally enters the human body, the gastric acid in the stomach has strong acid, which can kill bacteria. In the respiratory tract, such as nose hair in the nasal cavity and cilia in the trachea, foreign dust and germs can be blocked.

The urinary tract system is also a pipe connecting the body with the outside world, discharging excess water and waste in the body. The mucosa of urethra has a special pH value, which can prevent pathogens from invading the body. Moreover, repeated urination every day can wash away the bacteria accumulated at the urethral orifice and prevent bacteria from entering the human body. Under the protection of the front-line defense layer, everyone can carry out various activities without worry.

When the protection of the first line of defense fails, such as falling, skin injury, or colds and bacteria entering the respiratory tract, the body's second line of defense begins to start. At this time, the body begins to produce inflammatory reaction, which may be local redness and pain, or it may be symptoms such as fever all over the body. White blood cells in the body began to proliferate to fight the invaders. According to the different invasion sources, white blood cells will also assign different professional troops to resist, among which guerrilla soldiers who will devour germs include neutrophils, macrophages, natural killer cells and so on. However, cells attacked by germs will secrete interferon to form a protective cover, which can slow down the attack speed of germs in cells. In close combat with germs, you can also use supplements such as bone water to attach to germs and let them die quickly.

When no line of defense can stop the invasion of germs, the human body will carry out the third line of defense. The human lymphatic system produces two kinds of lymphocytes, one is T lymphocyte and the other is B lymphocyte. These two kinds of cells have different functions and functions. B lymphocytes are more like a central control center. Instead of hand-to-hand combat with pathogens, it produces a large number of antibodies in lymphoid tissues, which are transported to the front line of the battlefield through blood and combine with surface antigens on infected cells, so that pathogens can no longer play their roles. T lymphocytes are more like soldiers. When antibodies lock infected cells, antibodies and antigens combine to form "antigen complexes". At this point, the pathogen loses its toxicity, and T lymphocytes go to remove these antigen complexes. Once B lymphocytes produce antibodies, they will remain in the body for a period of time. As long as the same pathogen enters, antibodies are ready to deal with it. ? This is why as long as you have chickenpox, you can no longer be threatened by chickenpox, because you already have antibodies against chickenpox in your body, so you can fight back easily and won't get sick again. Vaccination also hopes to stimulate lymphocytes to produce antibodies with a small amount of viruses that will not cause physical diseases to prevent virus invasion and achieve preventive effects.