When a person is infected with a virus for the first time, the immune system triggers the production of antibodies that are specific to that virus. These antibodies have the power to attach to the virus so that the immune system can destroy it.
However, it takes a while for the body to produce these antibodies. They may not even show up until the illness has run its course and, hence, may only be useful if the virus infects an individual again. If antibodies are present, they will be found inside plasma or blood samples.
The COVID-19 IgG/IgM Rapid Test Cassette (Whole Blood/Serum/Plasma) Letter
of Authorization, along with the authorized Fact Sheet for Healthcare Providers, the
authorized Fact Sheet for Recipients, and authorized labelling are available on the
FDA website: FDA Website.