How does viruses affect the human body




















Therefore, T and B lymphocytes can specifically recognise foreign material, such as a viral particle, because of its unique antigens. In order to infect a cell, a SARS-CoV-2 virus particle must first bind to a specific protein called angiotensin-converting enzyme 2 ACE2 , which is present on the surface of cells in the lungs and other organs such the heart, kidneys and intestines.

Antibodies that bind to the viral spike protein can interfere with its ability to interact with ACE2 and therefore limit the ability of the virus to infect cells. During an infection, antibodies may be produced in response to any of the proteins that constitute the virus, but it is the antibodies that bind to the external spike protein that are most important in limiting virus spread and conferring immunity.

The relative importance of the different types of immune defence against viral infection depends on the type of virus. For an acute virus infection such as SARS-CoV-2, immune defences that slow its spread within the body are most important. The main immune defences against acute virus infection are depicted in Figure 3. In addition to their role in blocking infection, antibodies also promote removal and destruction of the virus by other cells of the immune system.

The production of antibodies that recognise particular viral antigens can persist for many months, which gives extended immunity against a virus, even when all the viral particles are cleared.

In the longer term, the production of antibodies that recognise the virus will decline, but the encounter with the virus is not forgotten. The immune reactions that occurred during the viral infection will have caused the population of lymphocytes that recognise that virus to expand. Some of these lymphocytes, referred to as memory cells , retain the ability to react quickly to a repeated infection with the same virus.

Hence, long-lasting immunity to a virus infection depends both on antibodies and the memory cells that recognise that virus. Long-lasting immunity to a virus infection depends both on antibodies and the memory cells that recognise that virus. It is important to note that this continued protection depends on the virus not mutating in such a way that it can evade the immune response.

Unfortunately, it is a characteristic of many viruses that they mutate their surface proteins so that the antigens they bear are different and the original antibodies are less effective against them. The mutation rates of RNA-viruses tend to be particularly fast.

However, mutation cannot be unlimited since a virus still has to bind to its host cell if it is to survive itself. The internal proteins within a virus are not normally accessible to antibodies, which bind onto the outside of the virus, and they are less susceptible to mutation. Cytotoxic T cells can recognise both the internal and external antigens of the virus provided they are presented on the surface of an infected cell.

Current evidence suggests that it is not a fast-mutating virus by comparison with for example HIV. In the early months of the pandemic mutations were detected in the spike protein, and two proteins involved in RNA replication, but the rate of mutation of these proteins appears to have slowed.

An important element of vaccine design is to initiate immune responses against parts of the virus that do not readily mutate. Ideally a vaccine will induce cross-reactive antibodies which will protect against most or all of the virus variants that might evolve. You may wonder what makes some individuals more susceptible to severe infections compared to others.

Genetic variation in the human population affects all aspects of the immune responses, described above. However, the immune response is only one component that determines severity of disease. Genetic factors that affect viral entry into the cell and replication are also important. There are several methods to determine whether a person has been infected with a virus, which essentially have two objectives:.

For an acute infection like SARS-Cov2, the virus is normally completely cleared from the body around the time that a person recovers from the illness. It is no longer detectable after this time Figure 4. Conversely, the antibodies that are produced as part of the immune response are first detectable as the patient starts to recover; their production gradually declines over the following months, although they may still be detectable years later.

How early the virus can be detected, and how late the antibodies can be detected partly depends on the sensitivity of the assays used. It is possible to detect a virus as an infection proceeds, but this is a lengthy procedure that must be done in specialised biological containment laboratories. Routine diagnosis of viral infection looks for viral nucleic acid, which constitutes the genetic material of the virus. It requires careful handling, since the swabs taken from potential patients may have infectious virus on them.

The characteristics of any organism is defined by the sequence of bases in its genome. The test for a virus detects just a unique segment of the entire viral genome. The first step is a chemical extraction that is often automated. The following two steps are carried out in another instrument called a PCR machine. The specificity of the qPCR i. These primers are carefully selected to amplify a segment of the specific viral genome and not DNA from the host or other pathogens that may be on the swab.

This procedure takes several hours, and there is no way that this can be reduced without compromising the sensitivity of the test. One way of reducing the time taken is to combine steps of the assay, or eliminate the need for separate isolation of the viral RNA. Another way of reducing the time is to reduce the number of amplification steps, but this then makes the assay less sensitive.

In a normal PCR reaction the samples must be heated, maintained at a high temperature, cooled, and then raised to an intermediate temperature. This cycle must be repeated times and that takes the time. In the LAMP technique some very clever molecular biology is used so that the reaction can proceed at one temperature isothermal amplification. Not only does this obviate the need for the lengthy temperature cycling steps, it can also be done with simpler instruments.

Nevertheless being able to reduce the test time to less than 1 hour has major advantages in some settings. Finally one can consider social aspects of testing.

It is much easier for people to provide their own nasal swabs or a saliva sample to test, but such samples are more likely subject to variability and saliva samples generally have less virus to test. This means that higher sensitivity is needed. You can see that there is a trade-off between speed, accuracy and convenience in the different tests. Different parameters are more important for each group of people.

For example, a rapid test might be of prime importance in an airport, whereas highest accuracy would be more important in a hospital. It is important to remember that in any individual there is a time-point when virus can be detected but antibodies have not yet been produced, and a later time period when the virus has been eliminated but the anti-viral antibodies remain. Although there are many elements to the immune response, detection of virus-specific antibodies is the only reasonable way to determine whether a response has taken place.

Antibody testing can be applied to large numbers of patients relatively easily. Antibodies appear in the blood as a person recovers from an infection. They may also appear in secretions such as saliva and tears, but detection in secretions is more difficult and may be less reliable. For this reason, the assays detect antibodies in blood serum; the fluid component of blood. As noted above, antibodies against the spike protein are most relevant to protection against coronavirus infection, but antibodies that recognise other viral proteins see Figure 2 may be used to establish whether someone has previously been infected with the virus.

For example, to detect antibodies against the spike protein, the spike protein itself would be used as the detection element in the ELISA.

There are now many companies trying to develop a lateral flow device similar to a home pregnancy test. Such tests could give a positive or negative result as to whether an antibody that recognises SARS-CoV-2 is present in the test fluid e.

A coronavirus, for example, has a sphere-like shape and a helical capsid containing RNA. It also has an envelope with crown-like spikes on its surface.

Seven coronaviruses can affect humans, but each one can change or mutate, producing many variants. Learn more about coronaviruses here. Just as there are friendly bacteria in the intestines that are essential to gut health , humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli.

Viruses do not leave fossil remains, so they are difficult to trace through time. Scientists use molecular techniques to compare the DNA and RNA of viruses and find out more about where they come from. Three competing theories try to explain the origin of viruses.

In reality, viruses may have evolved in any of these ways. The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites.

Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit. In this way, they gained the ability to become independent and move between cells. The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago.

When a viral disease emerges, it is not always clear where it comes from. A virus exists only to reproduce. When it reproduces, particles spread to new cells and new hosts. The features of a virus affect its ability to spread. Some viruses can remain active on an object for some time. If a person with the virus on their hands touches an item, the next person can pick up that virus by touching the same object. The object is known as a fomite. Viruses often change over time. Some of these changes are very small and do not cause concern, but others can be more significant.

Significant changes could make a virus more transmissible, as has been the case with the B. They may also help the virus evade the immune system or existing treatments. For example, doctors use several drugs in combination to treat HIV so that it is harder for the virus to develop resistance to treatment.

Influenza viruses can also do so-called antigenic shift. This can happen if a host cell has become infected with two different types of influenza virus. For instance, pigs can often serve as a mixing vessel for avian and human influenza viruses. Some viruses, such as HPV, can lead to cancer.

The full impact of a virus can take time to appear, and sometimes there may be a secondary effect. For example, the herpes zoster virus can cause chickenpox. The person recovers, but the virus may stay in the body. Years later, it may cause shingles in the same individual. Coronaviruses are a large family of viruses and include viruses that cause the common cold. However, it has changed many times since scientists first identified it in China.

By September , scientists had logged over 12, mutations, and the development continues. Some variants are more transmissible and more likely to cause severe illness than others. The main concern with new variants is the unpredictability of their impact. The main symptoms of COVID are dry cough , fatigue , and fever, but there are many possible symptoms.

In short, viruses can replicate and create other viruses. This is possible as they can adapt very easily to any environment and any host. They are made to survive very difficult conditions. Usually these microorganisms enter the body through the mouth, eyes, nose, genitals or through wounds, bites or any open wounds. Moreover, they are transmitted through different routes. Some diseases are spread by direct contact with infected skin, mucous membranes or body fluids.

There is also the possibility of indirect contact, when a person touches an object door, handle, table , which has the virus on it, when an infected person sneezes, coughs or talks or when the mucous membrane comes into contact with another person. In some other cases, the virus is transmitted through common vehicle such as contaminated food, water or blood.

Finally, there are vectors: rats, snakes, mosquitoes etc. These organisms enter the body and adhere to the cell surface. Depending on the type of virus, it seeks for cells in different parts of the body: liver, respiratory system or blood. Once it has attached itself to the healthy cell, it enters it. When the virus is inside the cell, it will open up so that its DNA and RNA will come out and go straight to the nucleus.

They will enter a molecule, which is like a factory, and make copies of the virus. These new copies of the virus millions of copies will leave the already infected cell to infect other healthy cells, where they will multiply again. Infected cells can be damaged or die while hosting a virus. It is important to clarify that when a virus infects a human, it does not always end up in a disease. The infection occurs when the virus begins to multiply. And, the disease occurs when many body cells are damaged by the infection, which is also when the symptoms and illness appear.

In a nutshell, if the immune system manages to fight off the virus that entered the cells and replicated, the person will not get sick.



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