Plasma therapy involves the transfer of the components of the adaptive immune system from a person who has recovered from an infection to another who is currently infected. Persons who have recovered from an infection have (usually) done so because their adaptive immune systems have found a way to effectively fight the invading pathogen. So transferring a small sample of the components of their adaptive immune system to an infected person can result in the treatment of the infection.
The components of the adaptive immune system like T cells, B cells, NK cells etc, are found in the blood plasma. Hence the term 'plasma therapy'.
For COVID-19, there are several studies going on but the results so far suggest that plasma therapy isn't very effective. You may want to read this.
https://www.cochrane.org/CD013600/p...recovered-covid-19-treat-individuals-covid-19
