NHS Blood and Transplant is boosting stocks of blood plasma for very ill coronavirus patients ahead of winter.
It wants more people who have recovered from Covid-19 to become donors.
Their plasma contains antibodies that are believed to help other sufferers fight the virus.
Fourteen new donation centres will open in November and December, to bring the total in England to 42. The blood plasma will be used to treat patients in Covid trials.
Mother-of-seven Ann Kitchen, from London, was the first person in the UK to get the treatment.
She was being treated for coronavirus in intensive care at St Thomas' Hospital.
NHSBT says donations are urgently needed to confirm the benefit for patients, and then make the treatment available for general use in the NHS.
About 850 people have received the transfusions so far.
Trial results are expected before the end of the year.
Minister for Innovation Lord Bethell said: "The use of convalescent plasma has shown promising results in treating coronavirus, and the opening of more donation centres is an important step towards getting this innovative treatment to more patients, if clinical trials demonstrate it should be available on the NHS.
"More donations are needed and everyone has a part to play in stopping this virus from harming our loved ones. I urge everyone who has had Covid-19 to come forward and donate - your contribution could save lives."
Donating takes about 45 minutes and requires the insertion of a needle into the arm.
After your appointment, you can get on with your normal day and your body will naturally replace the plasma you have donated.