Once penetrated into the host cell, SARS-CoV-2 triggers an extremely complex immune response from the infected organism. An important component of this immune response is the production of anti-virus antibodies, respectively antibodies specifically targeted on the structural proteins of the virus, explain the specialists. MedLife. What are these antibodies? As a structure, they are immunoglobulins belonging to classes A, M, G, D, E, and as a target, they are directed on proteins N, M, E and S.
The coronavirus genome encodes 4 main structural proteins: protein N (nucleocapsid), which is basically the nucleus of the virus comprising viral RNA, protein M (membrane), protein E (viral envelope) and protein S (spike), the surface protein that gives the “crown” appearance due to the trimeric structure.
Types of antibodies in SARS-CoV-2 virus infection: IgM and IgG
To date, only three classes of antibodies are studied in the dynamics of the immune response generally induced by viruses: antibodies of type A, M and G. For coronaviruses and, especially for SARS-CoV-2, the role of IgA antibodies is small. meaning. IgM and IgG antibodies have been studied since the onset of the pandemic and with the development of laboratory tests.
IgM – Imunoglobulina M is the type of antibody produced by the immune system immediately after infection. It is present in the first line of defense during viral infections, before the immune system produces IgG-type antibodies.
IgG – Imunoglobulinele G are extremely specific antibodies that are important for long-term immunity and immune memory.
It is now known that there is a significant difference in both the levels and the chronology of the appearance of these antibodies. If for most viruses, the first antibodies that appear are those of the IgM type, for SARS CoV-2 the situations in which the two classes of antibodies (IgM and IgG) appear and persist almost simultaneously are much more frequent.
What do anti-protein N (nucleocapsid) and anti-protein S (spike) antibodies mean?
Proteina N (nucleocapsida) is the protein that encapsulates the viral RNA and protects it from the host cell environment. In addition to its protective role, the N protein has the role of mediating the transcription of the viral genome, being essential in viral replication. This protein is strongly immunogenic, the antibodies directed against it being an important serological marker in highlighting the immune response to viral infection with SARS-CoV-2 virus.
Structure protein S (Spike) it is extremely complex. It has 2 subunits that make it possible to attach the virus to the cell, penetrate the virus into the cell and fuse it with the host cell. In other words, the structure of the Spike protein provides an important number of highly antigenic sites that determine neutralizing antibodies, respectively antibodies that prevent the attachment and penetration of the virus into the host cell.
There are studies that have shown that anti-N antibodies can be detected earlier, but persist a little less than anti-S antibodies. Anti-S antibodies could neutralize the infectivity of the virus, especially by blocking the RBD domain (host cell binding domain), which can give them the status of protective antibodies.
Equally, there are frequent situations in which, despite evidence of SARS-CoV-2 infection, the body’s antibody response is not demonstrated by any commercially available test, ie there are people who “do not develop antibodies.”
The information provided by the presence of either antibody, either anti-N or anti-S, whether IgG or IgM class, is that there was SARS-CoV-2 infection, even if it was asymptomatic or there were common symptoms, overlooked at the time.
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