The idea is that antibodies against a bacterium or virus remain in the blood of someone who has recently recovered from the infection.

Doctors used convalescent plasma as a treatment during the 1918 Spanish flu pandemic and had varying degrees of success with it during the 2003 SARS outbreak, the 2009 swine flu (H1N1) outbreak, and the 2014 Ebola outbreak in Africa. In 2015, scientists established this treatment as the protocol for Middle East respiratory syndrome (MERS).

Early in the COVID-19 outbreak, on January 20, 2020, doctors in China began treating severely ill patients with convalescent plasma. They reported encouraging results from five cases in the journal JAMA

https://www.medicalnewstoday.com/articles/covid-19-convalescent-plasma