To understand the immune system, we first look at the structure of the human body. First, the immune system in the human body is the most important defense system against the invasion of pathogenic bacteria. The immune system is composed of immune organs such as tonsil, thymus, spleen, lymph node, Peyer' s patches, appendix and bone marrow, immune cells such as platelets (IgG), lymph cell, mononuclear phagocytes, neutrophilic granulocytes, Basophil and Eosinophil, and immune molecules such as immunoglobulin, interferon, interleukin, tumor necrosis factor, cytokine, and complement.
It is amazing about this system that there is a set of natural defense mechanisms against pathogens on the human body: the first line of defense is the physical barrier such as skin, mucous membrane, and subcutaneous tissue. In short, the surface of the human body in contact with the external environment is covered with a complete layer of skin and mucosa. The skin is composed of multiple layers of flat cells, which can block the entry of pathogens. As long as the skin is not damaged, pathogens can’t invade. However, the mucosa, whose mechanical barrier effect is not as good as that of the skin, has only a single layer of columnar cells, but a variety of appendixes and secretions, such as ciliary movement of respiratory epithelium, ingurgitation of saliva and enterokinesia, which can drive out pathogens staying on the mucosa. Therefore, when the host is stimulated by cold air or harmful gases, the mucosal barrier of the upper respiratory tract is damaged, it is prone to tracheitis, bronchitis, pneumonia and so on. The skin and mucosa can secrete a variety of bactericidal and detoxifying substances. For example, the sweat glands of the skin can secrete lactic acid to make the sweat acidic (pH 5.2~5.8), which is not conducive to the growth of bacteria. Therefore, various bactericidal substances, secreted by mucosal glands in different parts, such as lysozyme, gastric acid and protease and fatty acids secreted by sebaceous glands have bactericidal and fungal effects.
The second line of defense in the immune system, various cellular defense mechanisms related to human immunity, can be divided into innate immunity and adaptive immunity, among which adaptive immunity is further divided into humoral immunity and cellular immunity. The main function is to deal with pathogens that want to invade the body, start the defense mechanism against the invading virus, and then start to deal with the invaders. The immune system is the most effective weapon for the body to defend against the invasion of pathogens. It can find and remove foreign bodies, pathogenic microorganisms and other factors that cause fluctuating internal environment. However, its hyperfunction will cause damage to its own organs or tissues. CD4 + T cells play an important role in many autoimmune diseases.
Next, T cells and B cells in immune system fight against the specific pathogens that first invade. T cells can produce memory T cells with immune memory: when the pathogen re-invades, memory T cells can enable the immune system to rapidly make an enhanced immune response. When pathogenic microorganisms such as bacteria and viruses enter the human body, the macrophages in the immune system first attack and phagocytize them. Then, through the function of enzymes, these bacteria that have been phagocytized and invaded are decomposed into fragments, and the fragments of these microorganisms are displayed on the surface of the macrophages to become antigens and then let the T cells in the immune system know about it. T cells react with the antigens of microorganisms immediately like the original locks and keys. At this time, macrophages will produce a substance of lymphokine to activate T cells. Once activated, T cells immediately notify the entire immune system of the invasion of the "enemy". At this time, the immune system will send out cytotoxic T cells and B lymphocytes, and finally produce specific antibodies through B lymphocytes.
Once the cytotoxic T cells find those infected human cells, they will destroy the infected ones like killers to prevent the further reproduction of disease-causing microorganisms. While destroying them, the antibodies produced by B lymphocytes and pathogenic microorganisms in the cells combine with knowledge to cure the disease. B cells, so-called memory B cells, also have immune memory in the immune system. The memory B cells can quickly produce IgG antibodies when the pathogen invades again. Therefore, after the first infection is suppressed, the immune system will record all the process tools of this pathogenic microorganism. If the human body is invaded by the same pathogenic microorganisms again, the immune system has clearly known how to deal with the invaders, and can respond easily, accurately, and quickly to eliminate the invaded place. Finally, threats and risks to health do exist in the internal or external environment. To deal with problems, the immune system needs to maintain high-efficiency operation without interruption so that it can respond promptly when a person’s health is under threats. However, the functioning of the immune system is interconnected and extremely complicated. Immunocompromised or overreacted can both cause an extra burden on the health. For example, allergies are usually caused by an overreaction of the immune system to external stimuli.
Finally, there are three fundamental functions of immune system:
(1) Defense: the immune cells protect the human body from viruses, bacteria, pollutants, and diseases.
(2) Attack: the immune cells fight viruses and diseases to remove infected cells.
(3) Repair: the immune cells can repair damaged organs and tissues to restore their original functions.
Usually, the functioning principle of the immune system is that when the pathogen penetrates the skin or mucosa and reaches the tissues in the body, phagocytes first escape from the capillary action to gather at where the pathogen locates. In most cases, pathogens are killed by phagocytosis. If not killed, the phagocytic cells in the lymph nodes further destroy the pathogens through the lymphatic vessels to the nearby lymph nodes. This filtering effect of lymph nodes plays an important role in the immune defense ability of the human body. In general, only highly virulent, and numerous pathogens that can’t be killed are likely to invade the blood flow and other organs but phagocytes in the blood, liver, spleen, or bone marrow, etc. are continuing to phagocytize and kill pathogens. However, when there is no threat of foreign pathogens in the human body, the existing energy for the immune system will automatically be converted into other forms for use by other organs or functions - turning attack into defense. On the one hand, the body's energy can be used in the proper place; on the other hand, it can help to increase the efficiency of defense, repair and reduce the chance of the human body being infected by environmental pathogens. In sum, to have a healthy body, we must have a sound immune system to operate normally. From the first line of defense, the pathogen is blocked outside the body, so that the virus won’t make trouble in the body, so as to prevent people from getting sick and achieve the goal of maintaining health. A healthy immune system is irreplaceable. Although its strength is amazing, it may still fail due to the continuous intake of unhealthy food. Therefore, proper nutrition can enable the immune system to operate fully and effectively to help the human body to better defend against diseases, overcome environmental pollution and the invasion of toxins. The inseparable and mutually reinforcing relationship between nutrition and the immune system creates another topic in nutritional immunology.
Reference: Immune System A + Hospital Encyclopedia
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