This is how your immune system reacts to coronavirus


Dana G Smith – PEOPLE infected with the novel coronavirus can have markedly different experiences. Some report having nothing more than symptoms of a mild cold; others are hospitalised and even die as their lungs become inflamed and fill up with fluid. How can the same virus result in such different outcomes?

Scientists are still perplexed by the novel coronavirus. But it’s becoming increasingly clear that the immune system plays a critical role in whether you recover from the virus or you die from it. In fact, most coronavirus-related deaths are due to the immune system going haywire in its response, not damage caused by the virus itself. So what exactly is happening in your body when you get the virus, and who is at risk for a more severe infection?

When you first become infected, your body launches its standard innate immune defence like it would for any virus. This involves the release of proteins called interferons that interfere with the virus’ ability to replicate inside the body’s cells. Interferons also recruit other immune cells to come and attack the virus in order to stop it from spreading. Ideally, this initial response enables the body to gain control over the infection quickly, although the virus has its own defences to blunt or escape the interferon effect.

The innate immune response is behind many of the symptoms you experience when you’re sick. These symptoms typically serve two purposes: One is to alert the body that an attack has occurred — this is thought to be one of the roles of fever, for example. The other purpose is to try and get rid of the virus, such as expelling the microscopic particles through cough or diarrhoea.

These very different symptoms emerge depending on where in the body the virus takes hold. The novel coronavirus gains entry into a cell by latching onto a specific protein called the ACE2 receptor that sits on the cell’s surface.

These receptors are most abundant in the lungs, which is why Covid-19 is considered a respiratory illness. However, the second-highest number of ACE2 receptors are in the intestines, which could explain why many people with the coronavirus experience diarrhoea.

The goal of the innate immune defence is to contain the virus and prevent it from replicating too widely so that the second wave of the immune system — the adaptive or virus-specific response — has enough time to kick in before things get out of hand. The adaptive immune response consists of virus-specific antibodies and T cells that the body develops that can recognise and more quickly destroy the virus. These antibodies are also what provide immunity and protect people from becoming re-infected with the virus after they’ve already had it.

In some people, however, the virus will replicate and spread rapidly before the immune system wrestles it under control. One reason this can happen is if a high quantity of viral particles infect the body — which is why doctors and nurses, who are exposed to huge amounts of the virus multiple times a day caring for patients, can have more severe infections even if they are young and healthy. The more virus there is, the harder it is for the immune system to manage.

Another reason the body can lose control over the virus lies in the immune system itself. The most vulnerable populations during the pandemic are elderly people, whose immune systems naturally start to decline with age, and people who are immunosuppressed because of another illness or medication. A suppressed immune system can result in a weaker initial interferon response or a delayed antibody response, allowing the virus to spread from cell to cell.

If the virus is able to take up residence in the lungs, the disease can progress to pneumonia as more cells become infected and inflamed. Part of the damage is caused by the virus, but an even greater amount is due to the immune system itself trying to destroy and get rid of those infected cells.

At this point, the disease can still go in two directions: The immune response can remain stable and regain control over the virus, eventually clearing it through T cell and antibody activity. Or the immune system can freak out and start to over respond, churning out more and more inflammatory proteins, called cytokines, in a frantic attempt to wipe out the virus. It’s this second path that causes substantial cell death in the lungs, resulting in the most severe infections, acute respiratory distress syndrome, and even death.

The elderly and immunocompromised are particularly vulnerable to this type of response as their underactive immune system suddenly kicks into hyper drive and becomes overactive

Most of the clinical trials conducted so far have involved treating these severe cases, which on the surface makes sense — you want to give the potentially effective drugs to the sickest people in case it can help save them.

For now, your best defence against the virus is to support your immune system with sleep, exercise, and good nutrition and, most importantly, to wash your hands and practice social distancing so you don’t get infected in the first place.

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