How does the human body prevent bacteria and viruses (1)?

In order to fight against diseases, the human body has established a complete and complex defense system. There are three main functions: first, to protect the human body from viruses, bacteria, mycoplasma and other pathogenic microorganisms; The second is to remove the waste after metabolism in time, clean up the battlefield after the war between human body and pathogenic microorganisms, and eliminate the mutated human cells; The third is to repair the damaged organs and tissues and restore their normal functions. The defense system of human body includes three levels, namely, physical barrier, nonspecific immune system and specific immune system. These defense systems are composed according to the system level. After the pathogen overcomes the first barrier, it immediately faces the second barrier. This multi-level system structure enables human beings to survive under the attack of thousands of pathogenic bacteria every day. Wan Li Great Wall of Human Body-Skin and Mucosa: Skin and Mucosa constitute the outermost natural barrier, and the human body can only communicate with the outside world through the four passes of mouth, nose, urethra and anus to complete energy metabolism. It is precisely because of the existence of these trading ports that pathogenic microorganisms are provided with ways to invade. The skin consists of many layers of flat cells, which can block the passage of pathogens. Only when the skin is damaged can pathogens invade. Many layers of flat cells in the skin are dead cells, which are colloid without metabolism. Because viruses can only invade the human body through living cells and reach the corresponding target cells, viruses such as influenza, measles and chickenpox cannot spread through intact skin. Sweat glands of the skin can secrete lactic acid to make sweat acidic, which is not conducive to bacterial growth. Fatty acids secreted by sebaceous glands have bactericidal and antifungal effects. Although the mucosa has only a single layer of columnar cells, the mechanical blocking effect is not as good as that of skin, but the mucosa has many weapons. The surface of respiratory tract is covered by epithelial cells, which not only have cilia structure, but also secrete mucus. The surface of the respiratory tract of healthy people can produce about 100 ml of mucus every day, which is not only beneficial to the movement of cilia, but also can adhere to pathogenic microorganisms and prevent them from entering cells. The cilia of epithelial cells have the function of active swinging, swinging in a certain direction, so that mucus and pathogens in mucus move towards the throat and are excreted by coughing and swallowing. Some mobile troops (phagocytes and white blood cells) swim in the mucus of some airways, which can eat pathogens at any time or limit the spread of pathogens. There are nose hairs in the front of the nasal cavity, which interweave into a large net to intercept most of the dust and larger bacteria; Cilia and mucus at the back of the nasal cavity can adhere to most pathogens and sweep them out of the house by sneezing and blowing your nose. The digestive tract is also an insurmountable barrier. Only the most drug-resistant pathogens can be infected through the digestive tract. The first barrier that pathogens encounter is the mouth. Oral saliva contains lysozyme, which can decompose bacteria efficiently. There are secretory leukocyte protease inhibitors in saliva, which can kill the virus and even have a good inactivation effect on HIV. Swallowing can also shorten the stay time of pathogens in the mouth, so that pathogens do not have enough reproductive time. After the pathogen is lucky enough to act on it through saliva, the pathogen will face the attack of gastric acid. Gastric acid can kill almost most microorganisms in food. There are many proteases in the stomach. These enzymes have the best activity under the action of gastric acid, and can degrade protein in food and protein of pathogenic microorganisms. There are a lot of digestive enzymes discharged from pancreas and bile salts discharged from liver in the duodenum, which can dissolve the protein shell and lipid of pathogens and kill these pathogenic microorganisms quickly. In addition, trillions of beneficial bacteria that have settled in the intestines have already carved up their territory, which can compete with pathogenic bacteria and make the arrested pathogenic bacteria have no place to live. Intestines and urethra excrete harmful bacteria and substances quickly through regular excretion. The natural immune system is the field army of the human body, exposed to an incalculable number of astronomical pathogenic microorganisms. We can't naively rely on the Great Wall of Wan Li to stop the invasion of pathogens. The main function of this physical barrier is to minimize the invasion of pathogens. Pathogens are too cunning to be intimidated by tall walls. Pathogens can also invade the human body through various channels. Please don't blame, pathogens are also for survival. If they don't infect humans or other animals, they can't survive, let alone reproduce, which will lead to the demise of the family. Pathogens are with human beings and will struggle with human beings for a long time. Without them, human beings will be lonely, defeated and will perish because of depression. In addition to the barrier of skin and mucous membrane, the human body also has a set of immune system, including natural immunity and acquired immunity. It is a national defense system for human body to resist pathogenic microorganisms, which consists of immune organs, immune cells and immune molecules. Immune organs include: bone marrow, thymus, spleen, lymph nodes, tonsils, small intestinal lymph nodes, appendix and so on. Immune cells include lymphocytes, mononuclear phagocytes, neutrophils, basophils, eosinophils, mast cells and platelets. Immunomolecules include: complement, immunoglobulin, cytokines, etc. The natural immune system consists of two major services, one is phagocyte system and the other is immunoglobulin system. When pathogens invade the human body, macrophages are the first to rush up. In peacetime, the main function of macrophages is to roam the tissues and collect garbage in the body, such as dead cell fragments, because a large number of cells die every day due to human metabolism. Once the pathogenic microorganisms cross the first barrier, macrophages will devour them mercilessly, but macrophages are equivalent to a moving whistle, which can only handle a small number of pathogens in most cases and play a more important role in informing the enemy. When the enemy situation is serious, it will quickly report to the headquarters to mobilize field troops, that is, white blood cells (phagocytes in prime of life include neutrophils and monocytes). That's why when we are sick, the doctor will give us a white blood cell test. If it is a bacterial infection, white blood cells will increase. So many people think that white blood cells are bad and a bad thing, which leads to illness. In fact, this is the result, not the cause. Leukocyte is a warrior against bacteria in human body, fighting bravely with the enemy until death. In the process of fighting the enemy, many kinds of hydrolases released by lysosomes in white blood cells can also destroy adjacent normal tissues and cells, causing adverse immunopathological damage to human body. This is a shortcoming, and we will talk about how to make up for it later. Neutrophils and monocytes in blood and macrophages in tissues and organs constitute a mononuclear phagocyte system. When the pathogen penetrates the skin or mucous membrane and reaches the tissues in the body, phagocytes first escape from the capillaries and gather at the location of the pathogen. In most cases, pathogens are swallowed and killed. If they are not killed, they will reach nearby lymph nodes through lymphatic vessels, where phagocytes will further destroy them. This filtering effect of lymph nodes plays an important role in human immune defense ability. Generally, only a large number of virulent pathogens can invade organs such as blood flow without being completely blocked. But phagocytes in blood, liver, spleen or bone marrow will continue to devour and kill pathogens. Bacteria are swallowed by phagocytes to form phagocytes; Lysosomes and phagocytes fuse into phagocytes; A variety of bactericidal substances and hydrolases in lysosomes kill and digest bacteria; Bacterial residues are excreted from cells. The Tao is one foot high and the devil is ten feet high. Some cunning pathogens just use the human body's phagocytic system to carry out ulterior secrets. Although they can also be swallowed by phagocytes, they cannot be killed. This is incomplete phagocytosis, such as tuberculosis, Brucella, typhoid and Legionella. Incomplete phagocytosis can protect these bacteria from specific antibodies, nonspecific antibacterial substances or antibacterial drugs in human body fluids. Some bacteria can also grow and reproduce in phagocytes, which leads to the death of phagocytes. Some phagocytes that can swim with it spread to other parts of the human body through lymph or blood flow, causing large-scale lesions. Immunoglobulin is a very important part of human natural immunity, which can be divided into three categories, namely immunoglobulin GIgG, immunoglobulin A(IgA) and immunoglobulin M(IgM). Human serum immunoglobulin IgG is the most important antibody in immune response, which exists only in monomer form. It is the main force in anti-infection, which can promote the phagocytosis of mononuclear macrophages, neutralize the toxicity of bacterial toxins and the combination of virus antigens, and make viruses lose the ability to infect human cells. The age of IgG synthesis in the body is later than that of IgM, and it begins to be synthesized in the third month after birth, and it is close to the adult level at the age of 3-5. It is the only immunoglobulin that can pass through the placenta, and it is the most important immunoglobulin within 6 months of a child's birth. IgA can be divided into serotype and secretory type, and serotype IgA can regulate cell phagocytosis. Secretory IgA(SIgA) is the main component of the body's mucosal defense system, covering the surface of nasal, pharyngeal, tracheal, intestinal and bladder mucosa. It can inhibit the attachment of microorganisms to respiratory epithelial cells and slow down the reproduction of viruses. It is an important barrier of mucous membrane, has antibody activity to some viruses, bacteria and antigens, and is the first line of defense to prevent pathogens from invading the body. When foreign antigens enter the respiratory tract or digestive tract, the local immune system is stimulated, and the immune response can be carried out by itself without the participation of the central immune system, resulting in secretory antibodies, namely SIgA. The content of SigA in respiratory secretions directly affects the resistance of respiratory mucosa to pathogens. IgM is the first immunoglobulin produced in humoral immune response induced by antigen stimulation, which is mainly distributed in serum. It is an efficient anti-biological antibody, and its bactericidal, bacteriolytic, phagocytic and agglutinative effects are 500- 1000 times higher than those of IgG. IgM plays an important role in the early defense of the body. Complement system is a protein reaction system composed of serum, tissue fluid and protein on the surface of cell membrane, which has a precise regulation mechanism. Complement system can participate in the specific and non-specific immune mechanisms of the body, which are mainly manifested as follows: 1. Dissolution and killing: when bacteria enter the body, lipopolysaccharide in bacterial cell walls can activate the complement system through replacement, leading to the destruction of bacterial cell walls. The second is immune adhesion: complement can concentrate the adhesion of small molecular pathogens, which is convenient for phagocytosis and elimination. Thirdly, chemotaxis: Complement can release signals when activated, so that white blood cells are concentrated to the location of pathogens, thus killing pathogenic microorganisms.