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Innate immunity key to children’s immune system still on top

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eMediNexus    23 September 2021

Immunity and COVID

Innate immunity is the first line of defense against the human immune system a plays a very important role in fighting various infections. To fight SARS-CoV-2 both the innate immune system comprising of granulocytes, monocytes, macrophages and NK cells. Different dendritic cells, innate lymphoid cells, or mast cells also belong to innate immunity. The adaptive immune system comprises of T and B cells, which play an important role. Recent studies have shown that several genetic findings, interaction maps of viral proteins with host factors and high-resolution single-cellomics analyses provide strong evidence that innate immunity played a key role in combating the COVID-19 infection.

The COVID-19 pandemic has affected all spheres of human beings globally. Compared to older people and adults, children have been affected less and the researchers and scientists give this credit to the innate immune system which is the nonspecific immunity present since birth. US Centers for Disease Control and Prevention reported that children and adolescents under the age of 18 accounted for 2% of hospitalizations due to COVID-19 across the country till August 2021. But with the emergence of the delta variant, concerns for the health and safety of children have increased. Reports from the US reveal that more children are getting infected by the delta variant. Data from the American Academy of Pediatrics state that till the end of last month, about 15% of the infected population were below the age group of 21 years. Though there is no clear evidence about children being more vulnerable to delta variant, yet cases seem to rise in some parts of the world. A survey from India concluded that more than half of the children aged 6-17 had a detectable amount of antibodies against SARS-CoV-2, which are produced after infection or vaccination. Some studies reported a rare condition in some infected children known as a multi-system inflammatory syndrome in children (MIS-C), which researchers say could be due to an outsized antibody or T-cell reaction to the infection. Also, some studies state that the Alpha variant underwent changes and caused suppression of the body’s innate immune response

But, some researchers have put forward that children may be having less number of ACE2 receptors, to which the SARS-CoV-2 virus binds to enter the host boy. In a study, researchers found that infants and teenagers could have a high viral load soon after being infected. Those children who suffered from the common cold, flu etc around the year and were exposed to other coronaviruses causing the common cold have antibodies that are ready with some defensive action and hence less affected till now due to the pandemic. Further, in another study, it was observed that overall the younger patients having milder symptoms when compared to older patients produced similar levels of antibodies. Also, there were reduced levels of specialized antibodies and cells related to adaptive immunity. Children showed low levels of neutralizing antibodies which prevent SARS-CoV-2 from infecting cells and white blood cells (regulatory and helper T-cells). 

The children included in the study had increased levels of the signaling proteins interferon-γ and interleukin-17, probably produced by the cells of the airways which mediate innate immunity. An increase in the signaling proteins interferon-γ and interleukin-17 indicates a pathogenic attack. Also, researchers showed that infected children had lower levels of inflammatory monocytes which act as a bridge between the innate and adaptive immune systems, but had increased levels of T follicular helper cells which provide early antibody protection. Further, studies showed that recently infected children had increased levels of activated neutrophils which ingest viral particles before they multiply. Few, researchers suggested that in these children, a significantly higher expression of genes encoding MDA5, a receptor was found, which is known to identify and recognize SARSCoV-2, as compared to adults. The moment, these receptors recognize the virus, it triggers the production of interferons, which acts as a defence against the virus from zero-day, which in adults is delayed and takes 2 days. Studies also suggest that children are also less susceptible to clot formation in blood vessels, which may act as an advantage in the protection against the COVID-19 infection.

References:

  1. Schultze JL, Aschenbrenner AC. COVID-19 and the human innate immune system. Cell. 2021; 184(7):1671-92.
  2. Mallapaty S. Kids and COVID: why young immune systems are still on top. Nature. 2021;597, :166-8.

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