How does the body’s immune system work against viruses?
February 24, 2020
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For the new coronavirus, due to the lack of antiviral drugs, symptomatic treatment (artificial lung breathing, parenteral nutrition, etc.) does not directly affect the virus.
The body really clears the virus and relies on the immune system to treat the virus. The killing is popularly called “immunity”.
Immunity is eloquent, well generalized, and widely accepted and widely used. But the term immunity is extremely abstract and vague, and it is not clear what the material basis is behind it. Therefore, when it is used, it is confused and doubtful.
So what exactly does immunity mean? Huang Bo, a professor from the Peking Union Medical College Hospital &. Chinese Academy of Medical Sciences explained in an article yesterday.
Defense against virus
As an extremely abstract concept, does immunity exist, and how to understand it? Take an ideal state as an example.
In a room where the new coronavirus was evenly distributed, 20 people entered the room and they were exposed to the virus for the same amount of time. It was observed that 10 people had no symptoms and 10 had symptoms. Further, among the affected people, 5 had mild symptoms and 5 had severe symptoms.
Since the number of viruses entering each individual is the same, then why are 10 people not getting sick and 10 people getting sick, and some of them are mild and some are severe?
This ability to defend and control virus invasion in the body is immunity.
Obviously, among the above-mentioned population, those who do not have the disease have stronger immunity than those who have the disease, and those with mild symptoms have better immunity than those with severe symptoms.
Therefore, the so-called “immunity” means, as the name implies, immunity from plague (mainly refers to bacterial and viral infections), and immunity from plague.
No matter how abstract the immunity is, when the virus invades the body, the body can mobilize a defense network like the three forces of the land, air and sea (the individual’s immunity is like the military power of a country), and eliminate the virus through multi-level, progressive defense channels By combining these pathways, immunity is formed.
Physical barriers form the first level of immunity
When a new type of coronavirus in the air enters the body through breathing, dense nose hairs are distributed in the nasal cavity, which directly blocks the invasion of the virus into the deep. At the same time, the virus will stimulate the nerve endings of the nasal mucosa, causing the body to sneeze. The mechanical invasion will exhaust the invading virus discharge.
This can explain that when people around you sneeze, you often say “Did you catch a cold” (the cold is mainly caused by a virus infection), and the scientific basis behind it.
In addition to the physical barriers in the nasal cavity, there is a large amount of mucus in the throat and trachea and bronchus. This mucus adhere to the virus and prevent the virus from infecting lung tissue cells.
In particular, the surface of the trachea and bronchus is a layer of mucous membrane composed of ciliated epithelial cells, mucus-secreting goblet cells, and immune cells. The smooth muscle layer is beneath the mucosal layer.
When the virus invades, goblet cells secrete mucus on the one hand, and smooth muscles strongly contract on the other hand, which produces symptoms of cough and sputum, and expels the virus wrapped in sputum.
As you grow older, goblet cells or smooth muscle cells may decrease their responsiveness to viral stimuli, reducing the protective nature of the physical barrier.
Complements in the blood form the second level of immunity
The virus must enter the cell to manifest its infectivity and virulence. Cells in the human body are not like on land. They are actually surrounded by liquid, and this liquid comes from the blood in the blood vessels.
In the blood, there is a protective system of proteins, called the complement system, that prevents the invasion of bacteria and viruses.
When the virus enters the cell, the complement system is activated, producing two antiviral effects:
The first is that complement mediates holes on the surface of infected cells and induces the death of infected cells. In the process of cell death, the viruses that enter them are also broken down and eliminated;
The second is that complement mediates the phagocytosis of infected cells by macrophages, thereby degrading the phagocytosed cells along with the virus inside the macrophages.
In addition, complement component C4 can neutralize some enveloped viruses and prevent them from entering cells. Complement components are mainly produced by the cells of the liver. When liver function is not good, the amount of complement production will decrease, thereby reducing the complement’s defense against virus invasion.
Lung epithelial cells themselves form a third level of immunity
Lung tissue epithelial cells express the ACE2 protein receptor. Via ACE2, new coronavirus enters normal lung tissue cells.
However, these cells do not respond to virus invasion and are left to be slaughtered by the virus. Instead, they inherit an ancient anti-virus mechanism that is commonly found in cells, that is, activating type I interferon (a core produced by cells Antiviral protein molecule).
When the virus enters the cell, type I interferon in the lung epithelium is activated, preventing the virus from replicating in the cell. This is a general response of the body’s cells to the invasion of the virus, and is the result of ancient cell evolution.
In individuals with weak immunity, if the production of type I interferon is low, the replication and expansion of the virus in lung epithelial cells will be unrestricted, resulting in a large number of virus particles replicating in infected cells.
Innate immune cells form the fourth level of immunity
The above three levels basically can only play a supporting role. To truly control the virus infection, we must rely on the body’s immune cells.
Immune cells can be divided into two categories, one of which is called innate immune cells (also known as innate immune cells or natural immune cells), including macrophages, dendritic cells, natural killer cells, etc. Play an important role in the process.
When the virus invades, while the first three defense pathways play a role, the innate immune cells also feel the stimulus of the virus and respond, including phagocytosis and degradation of virus particles by macrophages, and dendritic cells like plasma cells Millions of times at the site of infection produce release of type I interferon, killing of virus-infected cells by natural killer cells, and so on.
Different individuals and even the same individual under different conditions, the number of these innate immune cells distributed in the lung and respiratory tract and their functional status are different, so their defense effects against the virus are different.
B cells and T cells form the fifth level of immunity
Another type of immune cells is called acquired immune cells, that is, T cells and B cells. They are the main force and core force of anti-virus.
When the invading virus breaks through the mucous barrier of the respiratory tract, enters the lung epithelial cells, and is amplified and released in the infected cells, new virus particles can return with the lymph fluid and enter nearby lymph nodes (medically referred to as drainage) Lymph nodes).
The structure of the lymph node is like an orange, the outer surface is a film like orange skin, and the inside is composed of regions where B cells and T cells are gathered, such as the same small flap of orange flesh.
There is a space between the petals of orange flesh and orange peel, which are filled with lymph fluid, which is somewhat similar to the moat, and there are many B cells distributed on both sides of the channel.
When the virus reaches these sites with the lymph fluid, the virus will stimulate the B cells around the moat and produce many different types of antibodies, but only a few antibodies can recognize the virus, and most of them cannot recognize the virus. This is the so-called non-specific Heterosexual antibodies.
They can be produced in the early stages of viral infection. The purpose is to let those few antibodies that recognize the virus to help activate the complement pathway described above and phagocytosis of macrophages.
When the virus flows with the lymph fluid, crosses the moat and enters the area where the B cells of the orange flesh are located. At this time, the antibodies produced by the virus to activate the B cells are virus-specific, that is, these antibodies can recognize a certain component of the virus.
These early virus-specific antibodies belong to the IgM type and do not have a strong affinity (binding capacity) with the virus. Their role is to further enhance complement activation and phagocytosis of the virus by the phagocytic cells.
High-affinity antibody production takes about 2 weeks to reach its peak, which is the result of B cells undergoing full activation and mutation screening and transformation into plasma cells.
Plasma cells are very large and filled with newly synthesized antibodies. After the plasma cells release the antibodies, the antibodies enter the blood and reach the site of viral infection through the blood circulation. Its main role is to bind the virus particles and prevent the virus from invading the cells.
However, antibodies are helpless for viruses that have entered cells, and the ultimate killing of viruses that hide in cells depends on T cells in the human body.
Viral-infected respiratory tracts recruit a class of dendritic cells with special functions through inflammatory factors. They reach the virus-infected site, take up the viral antigen (viral protein), and return to the area of lymph node T cell aggregation through lymphatic fluid for T Cells to recognize viral antigens.
T cells that can recognize viral antigens are activated and expanded in large numbers (100,000 T cells can be expanded). This is the activated virus-specific T cells. They then leave the lymph nodes, enter the blood, and circulate through the blood. Enter virus-infected respiratory tracts and identify virus-infected cells to kill them.
At the same time, the virus in the cell is also degraded and eliminated with death.
T cell killing ability is very powerful. One T cell can kill many infected cells in succession. Therefore, the ultimate control of virus infection depends on T cells.
The generation of antibodies and virus-specific T cells is the check level and the last level of the body’s antiviral immunity. If the previous four levels are not able to control the virus.
The last level will be to control the virus’s last killer, that is, antibodies prevent the virus from entering the cell, and for viruses that have entered the cell, they will be cleared by T cell killing.
However, the production of antibodies and the large number of virus-specific T cells depend not only on the number and status of T and B cells and the amount of viral antigens exposed, but also on the regulation of a series of factors around T and B cells. Differences in these areas lead to differences in antibody production and T cell activation among different individuals.
Lack of immunity evaluation indicators
As mentioned above, the body’s immunity against the new coronavirus is not composed of a single factor, but rather the physical barrier of the respiratory tract, the complement system in the blood, the interferon pathway of the lung epithelial cells, the innate immune system, and the acquired immune system And other five aspects together.
Each of these five aspects is important, but it is clear that their contribution is unequal, and we are not clear about the exact contribution of each aspect.
Even if we know its contribution and how to evaluate each aspect, there are currently no corresponding means and evaluation indicators.
For an individual, the thicker the nose hair and the more mucus secreted by the mucous membrane, the easier it is to expel the virus physically;
Its healthy liver function and complete complement system help prevent the virus from invading and clearing the virus by phagocytes;
The lung epithelial cells can quickly synthesize higher levels of type I interferon after the virus enters, and the antiviral effect is good;
The faster the plasma cell-like dendritic cells respond and the greater the amount of type I interferon released, the stronger the phagocytosis of macrophages, and the better the killing activity of natural killer cells against virus-infected cells, the better the effect of suppressing the virus;
Finally, the higher the affinity and amount of antibody produced, the earlier and the greater the number of activated virus-specific T cells, the better the effect of virus control.
However, before virus infection, how to predict the degree to which the individual’s different levels of responsiveness to the virus will reach is a great challenge facing the field of immunology.
How to improve anti-virus immunity?
Although antiviral immunity is composed of the different levels mentioned above, it is the cells that determine the level of individual immunity.
For example, if liver cells are healthy, the amount of various complement proteins produced is sufficient;
Normal lung epithelial cells can effectively produce type I interferon;
It is particularly important that bone marrow cells function better because all immune cells are produced by bone marrow hematopoietic stem cells. Only bone marrow cells are good, and the immune cells generated by their differentiation are viable.
For adolescents and young people, their liver cells, bone marrow cells, and lung epithelial cells are in good condition, so their immunity is also good.
However, as age increases, the body’s functions begin to degrade, and the state of liver cells, bone marrow cells, and lung epithelial cells all begin to decline, so antiviral immunity also begins to decline.
Therefore, the elderly need to pay special attention to improving their anti-virus immunity.
Regular exercise can better improve glucose and lipid metabolism, not only enhance liver cell function, but also improve the functions of bone marrow cells, immune cells, and lung epithelial cells;
The body’s immune cells are regulated by the neuroendocrine system. A pleasant mood helps the body’s immune cells to be active, while depressed mood and irritability inhibit immune cell function;
In the end, a healthy and reasonable diet is undoubtedly the basis for maintaining normal cellular function.
In the face of the current epidemic, eating more foods such as mushrooms, wolfberry, ganoderma powder, and black fungus can help improve immunity, because these foods are rich in plant polysaccharides (shitake mushroom polysaccharides, wolfberry polysaccharides, Ganoderma polysaccharides, etc.) Receptor molecules that can stimulate the surface of natural immune cells put these immune cells in a pre-stimulated state.
Immunity, as a body’s ability to ward off plague, has been widely used by the general public, especially the outbreak of the new coronavirus, which has made it widely spread in the news media.
For such an extremely abstract and ambiguous term, we need to understand what its real meaning is and what its material basis is. Only in this way can we take corresponding strategies to improve our body’s anti-virus immunity.