Antibody tests can tell us whether someone has had an infection in the past, even if they have not had symptoms. Also known as serological tests and usually done on a blood sample, these tests detect antibodies produced in response to an infection. In most people, antibodies start to develop after days to weeks and can indicate if a person has had recent (IgM type antibodies) or past infection (IgG type). Antibody tests cannot be used to diagnose COVID-19 in the early stages of infection or disease. They also cannot alone confirm immunity or duration of protection from reinfection.
Source - WHO