Why do some vaccinated people have 'low' Covid antibodies? What is immunosuppression?

Some 40% of people who have impaired immune systems have a “low” antibody response after being double-vaccinated

Some 40% of people who have impaired immune systems have a ‘low’ antibody response after being double-vaccinated (PA)

What is immunosuppression and why have some people been found to have “low or undetectable” levels of antibodies, even after getting both Covid-19 jabs?

What has the UK's latest Covid-19 trial found?