With the multitude of antibody tests for COVID-19 that are available, it is essential for healthcare workers to be aware of the tests available and how they can be used effectively.
The COVID-19 virus has four main structural proteins: nucleocapsid (N), spike (S), membrane (M) and envelope (E). The S protein consists of the 51 and 52 subunits. The S protein is highly immunogenic since it is located on the surface of the virus.”
The N protein plays an important role in the transcription and replication of viral RNA, packaging the encapsulated genome into virions and inhibits the cell cycle process of the host cells. The N protein is abundantly expressed during infections and also has high immunogenic activity. Therefore, both Nand S protein could be potential targets for the antibody-based detection of SARS-CoV-2.” However, evidence indicates that antibodies to the viral nucleocapsid may decline faster than those to S1 RBD or to the entire spike protein.
The S1 subunit contains immunologically crucial receptor binding domain (RBD), which is the key target of neutralizing antibodies.
Humans produce antibodies against the spike protein and S1 RBD, as well as other viral proteins. Neutralizing antibodies form a subset of the total antibody response. These antibodies can inhibit viral replication and represents a major mechanism of humoral immunity against viral infection. Multiple studies indicate a primary role for neutralizing antibodies that target the spike protein of SARS-CoV-2, with antibodies to the S1 RBD estimated to comprise ~90% of neutralizing activity.
This test is for the qualitative detection of the total antibody load against the SARS-CoV-2 Virus, which is the earliest and most sensitive serological marker and which becomes detectable from the second week of symptom onset.” However, since this test detects all antibodies (IgM, IgG & IgA) against the N protein, which as mentioned previously is highly immunogenic, it is ideal to detect the immune response post an infection.
These tests are for the qualitative detection of IgM and IgG antibodies produced against both the nucleocapsid and spike protein (whole). Since it also detects antibodies against the spike protein, IgG detection can be representative of protective neutralizing antibodies.