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Pre-existing memory T-cells may reduce Covid-19 severity

Date: 31 August 2020 Tags: Miscellaneous

Issue

Researchers have found that 20-50% people who have not been infected with novel coronavirus (SARS-CoV-2) still harbour memory T cells derived from previous exposures to common cold coronaviruses.

 

Background

They established the presence of memory T cells from common cold coronaviruses in people who have not been exposed to SARS-CoV-2 virus and found the memory T cells cross-reacting with novel coronavirus.

 

Details

  • It is possible that people with a high level of pre-existing memory CD4+ T cells that recognise novel coronavirus could mount a faster and stronger immune response upon exposure to the virus and thereby limit disease severity.

  • Researchers recorded robust memory T cells responses even in healthy people. Based on this they noted that memory T cells indicates a previously unanticipated degree of population-level immunity against COVID-19.

  • The cross-reactive memory T cells on activation would help in the development of plasma cells and thus antibody production, and in the development of killer T cells that would kill virus infected cells.

  • The latter reduces the reservoirs of infection. This would most likely reduce disease severity.

  • It is not yet known if such cross-reactive activation would have effects on the cytokine storm that causes most of the pathology and mortality in severe COVID-19 cases.

  • Antibodies protect against extracellular pathogens. But since antibodies cannot enter cells, these are unable to destroy cellular reservoirs of infection. 

  • A few researchers have also been mistaking pre-existing cross-reactive memory T cells to be playing a part in achieving herd immunity even when only a small percentage of the population is infected.

  • Memory T cells would not provide protective immunity from infection but may mitigate or reduce severity of disease. 

  • T cell immunity will not generate herd immunity, but there is a belief that there could be an unanticipated degree of immunity against more severe forms of COVID-19 in the population.