Immunoglobulin is used to treat viral infection. There are different immunoglobulin already used for the treatment of different viral diseases. Whether immunoglobulin is available or not against the corona virus.
The problem is if the adaptative immune system of the host (in thaht case, a human being with covid-19 infection) could produce permanently specific antibodies (A.K.A. immunoglobulins) vs COVID-19. Even if we treat patients with such therapy (meaning that we already discover the right type of antibody who bind a specific epitope present on the virus envelope or on the cell membrane the virus targets as host, due to his mechanism of replication) we may trigger an immune response by macrophages and limphocytes and even heal the patient, but is not guaranteed it would be permanently immunized (or, at least transiently for a long time, like the 10 years immunitary response we get after an antitetanus vaccination).