Many people experienced serious Covid-19 infection leading to hospitalisation and even death, while some people escaped with milder symptoms.

Researchers from the Stanford University in the US showed that in such people, the immune cells are better equipped to mobilise quickly against SARS-CoV-2, the virus responsible for Covid-19.

The immune cells, called killer T-cells, roam through the blood and lymph, park in tissues and carry out stop-and-frisk operations on resident cells.

The study, published in the journal Science Immunology, showed that killer T-cells taken from the sickest Covid-19 patients exhibit fewer signs of having had previous run-ins with common-cold-causing coronaviruses.

Many of these killer T cells were in "memory" mode, said Mark Davis, Professor of microbiology and immunology at the Stanford's School of Medicine.

For the study, the team analysed blood samples taken from healthy donors before the Covid-19 pandemic began, meaning they'd never encountered SARS-CoV-2 -- although many presumably had been exposed to common-cold-causing coronavirus strains.

They found that Covid-19 patients with milder symptoms tended to have lots of killer-T memory cells directed at peptides SARS-CoV-2 shared with other coronavirus strains.

Sicker patients' expanded killer T-cell counts were mainly among those T-cells typically targeting peptides unique to SARS-CoV-2 and, thus, probably had started from scratch in their response to the virus.

"It may be that patients with severe Covid-19 hadn't been infected, at least not recently, by gentler coronavirus strains, so they didn't retain effective memory killer T cells," Davis said.

(This story was published from a syndicated feed. Only the headline and picture has been edited by FIT)