Convalescent plasma has been used to treat infections for at least a century, dating back to the 1918 Spanish flu pandemic.
It was also trialed during the 2009-2010 H1N1 influenza virus pandemic, 2003 SARS epidemic, and the 2012 MERS epidemic.
Convalescent plasma was used as a last resort to improve the survival rate of patients with SARS whose condition continued to deteriorate.
It has been proven 'effective and life-saving' against other infections, such as rabies and diphtheria, said Dr Mike Ryan, of the World Health Organization.
'It is a very important area to pursue,' Dr Ryan said.
Although promising, convalescent plasma has not been shown to be effective in every disease studied, the FDA say.
Is it already being used for COVID-19 patients?
Before it can be routinely given to patients with COVID-19, it is important to determine whether it is safe and effective through clinical trials.
The FDA said it was 'facilitating access' for the treatment to be used on patients with serious or immediately life-threatening COVID-19 infections'.
It came after New York Governor Andrew Cuomo said that plasma would be tested there to treat the sickest of the state's coronavirus patients.
COVID-19 patients in Beijing, Wuhan and Shanghai are being treated with this method, authorities report.
Lu Hongzhou, professor and co-director of the Shanghai Public Health Clinical Centre, said in February the hospital had set up a special clinic to administer plasma therapy and was selecting patients who were willing to donate.
'We are positive that this method can be very effective in our patients,' he said.
Meanwhile, the head of a Wuhan hospital said plasma infusions from recovered patients had shown some encouraging preliminary results.
The MHRA has approved the use of the therapy in the UK, but it has not been revealed which hospitals have already tried it.
How does it work?
Blood banks take plasma donations much like they take donations of whole blood; regular plasma is used in hospitals and emergency rooms every day.
If someone's donating only plasma, their blood is drawn through a tube, the plasma is separated and the rest infused back into the donor's body.
Then that plasma is tested and purified to be sure it doesn't harbor any blood-borne viruses and is safe to use.
For COVID-19 research, people who have recovered from the coronavirus would be donating.
Scientists would measure how many antibodies are in a unit of donated plasma - tests just now being developed that aren't available to the general public - as they figure out what's a good dose, and how often a survivor could donate.
There is also the possibility that asymptomatic patients - those who never showed symptoms or became unwell - would be able to donate. But these 'silent carriers' would need to be found via testing first.
Japanese pharmaceutical company Takeda is working on a drug that contains recovered patients antibodies in a pill form, Stat News reported.
Could it work as a vaccine?
While scientists race to develop a COVID-19 vaccine, blood plasma therapy could provide temporary protection for the most vulnerable in a similar fashion.
A vaccine trains people's immune systems to make their own antibodies against a target germ. The plasma infusion approach would give people a temporary shot of someone else's antibodies that are short-lived and require repeated doses.
If US regulator the FDA agrees, a second study would give antibody-rich plasma infusions to certain people at high risk from repeated exposures to COVID-19, such as hospital workers or first responders, said Dr Liise-anne Pirofski of New York's Montefiore Health System and Albert Einstein College of Medicine.
That also might include nursing homes when a resident becomes ill, in hopes of giving the other people in the home some protection, she said.