Each year, between 5 and 20% of the population of the United States is infected with the influenza virus. On average, around 200,000 people they need to be hospitalized and up to 50,000 die. Those over 65 They are more likely to get the virus, because the immune system weakens with age. In addition, the elderly they are more likely to suffer a chronic disability after passing the flu, especially if they are hospitalized. ( Editor's note: In Spain, data on the situation and evolution of this disease are offered by the Influenza Surveillance System in Spain).
The flu symptoms They include fever, cough, sore throat, muscle and headaches, and fatigue. But what is causing all this chaos? What happens in our body while we fight the fever?
I am a researcher specializing in immunology at the University of Connecticut School of Medicine, and my work in the laboratory focuses on knowing how the infectious flu affects the body and how it fights against the virus. It is interesting to note that many of the defenses of our body responsible for attacking the virus are also causing many of the symptoms associated with influenza.
How does fever break out in your body?
The flu causes an infection in the respiratory tract, or what is the same, in the nose, throat and lungs. The virus is usually inhaled or transmitted through the fingers, the mucous membranes of the mouth, nose or eyes. After overcoming the respiratory tract, it joins the epithelial cells that line the airways of the lung by means of molecules of the cell surface. Once inside the cells, the virus "hijacks" the protein production system to generate its own viral proteins and create more infected particles. When the particles containing the virus reach their peak they are released and can invade adjacent cells.
While this process causes injury to the lung, most flu symptoms are caused by the immune response to the virus. This initial response involves cells of the innate immune system, such as macrophages and neutrophils. These cells contain receptors that are capable of perceiving the presence of the virus. When they do, they emit a signal producing small molecules similar to hormones, called cytokines Y chemokines, that alert the body that an infection exists.
The cytokines group other components of the immune system to fight correctly against the virus, while the chemokines direct these components to the place where the infection is located. One of the cells that come into play are the so-called T lymphocytes, a type of white blood cell that fights infection (sometimes called "soldier" cells). When these components identify the proteins of the influenza virus, they begin to proliferate in the lymph nodes that surround the lungs and populate the throat, which causes swelling and pain in the lymph nodes.
After a few days, the T cells move to the lungs and kill the cells infected with the virus. This process generates considerable lung damage, similar to bronchitis, which could worsen a hypothetical organ disease and hinder breathing. In addition, the appearance of mucosa in the lungs as a result of the immune response to the infection produces an annoyance cough reflex to try to clear the airways. Normally, the damage inflicted by the arrival of T cells to the lungs is reversible in a healthy person, but if it persists it can even lead to death.
The proper functioning of T cells that specifically fight against influenza is essential for the complete elimination of the virus from the lungs. When its effectiveness declines, which usually occurs at advanced ages or during treatment with immunosuppressive drugs, viral recovery is delayed, which translates into prolonged infection and increased lung damage. It can also pave the way for the appearance of complications such as bacterial pneumonia secondary, which can often be deadly.
Why does the head hurt so much?
Although under normal circumstances the influenza virus is confined completely to the lungs, several of its symptoms (among which are fever, headache, fatigue and Muscle pain) are systemic.
To counteract the flu infection, the cytokines and chemokines produced by the innate immune cells in the lung become systemic, that is, they enter the bloodstream and contribute to the onset of these systemic symptoms. When this happens, a concatenation of complicated biological events occurs.
One of the things that happens is that the interleukin-1, a type of inflammatory cytokine of great importance for the development of the T cell response that kills the virus. However, it also affects the part of the brain in the hypothalamus that regulates body temperature, causing fever and headaches.
Another transcendental cytokine to eliminate influenza is the so-called " tumor necrosis factor alpha" This protein can have direct antiviral effects, which is a good thing, but it can also cause fever, loss of appetite, fatigue and weakness during the flu and other types of infections.
What is the origin of muscle pain?
The research conducted has also discovered another aspect that tells us about the way in which influenza infection affects our body.
It is well known that muscle pain and weakness are two of the most recognizable symptoms of influenza. A study carried out with an animal revealed that influenza infection leads to an increase in gene expression that degrades muscle building, as well as to a reduction in gene expression that favors muscle building in the leg muscles.
The flu also makes walking difficult and weakens the strength of the lower extremities. It is important to note that in young individuals these effects are transient and normality returns once the infection disappears.
On the contrary, the impact of the flu can persist in the elderly. It does not stop being important, since a decrease of the stability and the strength of the legs supposes that the old ones are more prone to suffer falls during the recovery. It could also lead to a chronic disability that would generate the need to use a cane or a walker, which would limit the mobility and independence of the patient.
Researchers working in my laboratory believe that the impact of influenza infection on muscles is another of the involuntary consequences of the immune response to the virus. We are currently focused on determining what specific factors produced during the immune response are responsible for muscle pain, and we want to find out if we can find a way to prevent it.
In short, if you feel sick when you have the flu, do not worry. That means that your body is fighting the spread of the virus in your lungs and killing the infected cells. That is, he is fighting hard.
Article translated thanks to the collaboration of Lilly Foundation