Thank you to Eshani M King for allowing Children's Health Defense to run her letter in response to Dr. Peter Doshi's article — Covid-19: Do many people have pre-existing immunity? — that was featured in The BMJ last week.

Dear Editor,

Thank you to Dr. Doshi for raising the profile of T-cells. Incidentally, German researchers found that a staggering 81% of individuals had pre-existing T-cells that cross-react with SARS-CoV-2 epitopes. This fits with modelling in May by Imperial College's Professor Friston, a world authority in mathematical modelling of complex dynamic biological systems, indicating that around 80% and 50% of the German and UK populations, respectively, are resistant to COVID-19.

Antibodies can only latch onto and help destroy pathogens outside cells and may also occasionally, paradoxically, enhance a pathogen's ability to infect cell instead by antibody dependent "enhancement" or ADE. It is only the T-cell that can cleverly sense and destroy pathogens inside infected cells using "sensors" which detect foreign protein fragments.