COVID disease. What are antibodies to coronavirus and how do they work?

As a structure, the antibodies 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 practically 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 appearance of “crown” due to its structure trimerice.

Antibodies specific for SARS-CoV-2 virus infection: IgM and IgG

Until the present, 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. According to MedLife specialists, for coronaviruses and, especially for SARS-CoV-2, the role of antibodies of type IgA is poorly understood. IgM and IgG antibodies have been studied since the onset of the pandemic and with the development of laboratory tests:

• IgM – Immunoglobulin 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 – Immunoglobulins 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 are anti-protein N (nucleocapsid) and anti-protein S (spike) antibodies?

Protein N (nucleocapsid) is protein that encapsulates 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.

The structure of the S protein (Spike) is extremely complex. It has two 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 virus from attaching and entering the host cell.

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 they 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.

How can the presence of anti-protein N and anti-protein S antibodies be detected in the blood?

• Test anticorpi IgM anti-S – qualitative test, recommended for people who have been vaccinated and are not sure that they have suffered from COVID-19 disease.

• Test anticropi IgG anti-N – qualitative test, recommended for people who have not been vaccinated and are not sure that they have suffered from COVID-19 disease.

• Test anticorpi IgG anti-S – quantitative test, recommended for people who have been vaccinated and are not sure that they have suffered from COVID-19 disease.