THE new coronavirus pandemic has created a tremendous threat to global health and well-being. The highly contagious virus, SARS-CoV-2, caused illness in more than six million people in as many as 213 countries. Such a huge number of cases and death created fears ion all. The fears have now become an integral part of our daily conversations. The role of the body’s immune system in combating the virus, however, remain mostly unaddressed. Although the virus can be deadly, the vast majority, more than 80 per cent, of those infected experience only mild symptoms of cold and fever. A fraction of the rest experience severe respiratory problems and may need to be cared for in hospital with supportive treatment and may even die.
How can the same virus result in such different outcomes? It may, however, be linked with individual’s immune status, which can be strong or weak depending on the type, amount and the route by which the virus enters the body. The immune system has so far shown its potentiality in overcoming all the past pandemic occurrences in history when the vaccines were not available. Until and unless we have an effective vaccine to overcome the present pandemic, a key player in fighting against the novel coronavirus would, therefore, be our immune system. It would protect us against the deadly invader and can even be helpful as potential therapy.
Shortly after the viral attack, the body’s immune system starts operation with its soldier-like cells equipped with armament. The soldier cells that come forward first for the battle are mainly granulocytes, macrophages and natural killer cells. These cells take part in an operation, known as the innate immune response, in which they mainly fight SARS-CoV-2 using toxic enzymes, interferons, secreted by infected cells, and other potential signalling molecules. Natural killer cells particularly kill virally infected cells after specifically recognising stress proteins expressed on the infected cell surface. The innate immunity plays a key role to prevent the virus from replicating too widely. The symptoms of COVID-19 remain, therefore, mostly unnoticed in majority of the population.
On the other hand, SARS-CoV-2 uses various tricks to bypass and evade the immune system’s defensive operation. After a time delay, the virus faces a second wave of defensive operations known as the adaptive immune response. As the infection goes further, cytotoxic T-cells get expanded and they keep patrolling to look for the virally infected cells. Once a cytotoxic T-cell finds an infected cell, it typically latches on and fires toxic molecules that punch through the infected cell’s membrane. As a result, the infected cell along with its reserves of virus is destroyed. B cells, on the other hand, form antibodies that can keep the virus in check by binding its spike protein specifically. The virus loses its infectivity after its spike proteins are masked with antibodies. Our immune system also memorises the invader and it would produce antibodies quickly to restrict that virus if it becomes re-infected with the same virus.
Apart from triggering a normal immune response, sometimes the body’s response to new coronavirus infection can go into overdrive. This happens when the virus takes up firm residence in lung cells and the immune system tries to destroy and get rid of the infected cells. An overproduction of immune cells and their activating compounds, cytokines, may occur in such patients, who may show a cytokine storm syndrome. The elderly and/or immuno-compromised individuals with existing conditions such as diabetes and cardiovascular disease are particularly vulnerable to this type of response. It is only because their underactive immune system suddenly becomes overactive.
The cytokine storm is associated with hyper-inflammation due to a surge of activated immune cells in the lungs. The violent battle between the virus and the immune cells results in the piling up of dead cells in the lungs which clog the airways and reduce oxygen flow. This eventually worsen a patient’s respiratory function and cause multi-organ failure, which is frequently fatal. The exaggerated host response is almost invariably responsible for this type of fatal cases. It should, therefore, be kept in mind that a proper immune response saves lives; an excessive and prolonged response may, however, terminate lives as well.
Another important aspect of the components of immune system is its possible therapeutic application. The antibodies generated in a recovered patient can be explored for the treatment and prevention of COVID-19. Until an effective vaccine is developed, convalescent plasma therapy can be a quick alternative treatment option for severely affected patients. This is a process of passively immunising a severe COVID-19 patient using the plasma of a recovered COVID-19 donor. The antibodies present in the plasma of recovered donor will neutralise the patient’s virus and reduce its infectivity. In past few days, a number of hospitals in Dhaka has conducted plasma therapy that opened a promising alternative to treating the pandemic viral disease.
On the other hand, UK scientists recently found a sort of immune clue that sparked treatment hopes for severely ill COVID-19 patients. The scientists observed that the patients with the most severe form of the disease have an extremely low number of T-cells that made them incapable of fighting against the virus. The UK doctors became encouraged when they observed the recovery of those patients after they had been treated with interleukin 7, a drug that boosted the number of their T-cells. As the T-cell count increased, the patient again became able to mount an effective immune response to get the viral infections cleared. Plasma therapy and interleukin-mediated T-cell boosting — the two immunity-related treatment options — are, therefore, generating a ray of hopes for treating the disease in the absence of an effective vaccine or an antiviral drug.
The immune system fights against all pathogenic invaders. It is, therefore, better to reduce the burden of the immune system from battling against SARS-CoV-2 by maintaining a strict social distancing culture. However, until and unless an effective vaccine or an antiviral drug is developed, we must remember that a key player to fight against the virus is our immune system.
Dr Md Anwarul Azim Akhand is a professor of genetic engineering and biotechnology in the University of Dhaka.
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