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About Spike protein of SARS CoV 2

Spike protein is one of the structural proteins of SARS CoV 2 which is causative pathogen of Covid-19. SARS CoV 2 is an enveloped virus with a positive sense single stranded RNA. Spike proteins are present on the surface of the virus, it is called so as it appears as spikes in electron microscopy. Spike proteins are required for the virus to attach to the host cell. Spike protein has two regions S1 and S2, S1 region has a domain called Receptor binding domain (RBD) which will attach to the ACE 2 receptor on the host cell.


Antigenic Importance of Spike Protein

Spike protein is a major antigen of the virus which is recognized by our immune system and antibodies are produced against them, which will inhibit the binding of the virus, and as well as facilitate our immune cells to destroy the virus. Most of the vaccines are developed to target the spike protein of the virus.


What is the significance of Spike Protein Antibody Testing? Why is it required?

Spike Protein antibody testing are mainly used for the serosurveillance of the population to understand the epidemiology of the virus. The assays for spike protein antibody test mainly detects the antibodies against S1 region or the RBD domain of the spike protein.
 
The antibody response is varied among the population. It starts appearing within few days after the infection, might be negative in the initial or acute stage of infection and may remain for a longer duration after the infection. Spike protein antibodies test is also supporting the diagnosis of the COVID-19 complication like multisystem inflammatory syndrome or another sequel of COVID-19.


Interpretation of Spike Protein Antibody Test

Presence of spike protein antibodies indicates either presence of infection, immunity developed after infection or immune response for covid vaccination.

Presence of spike protein antibodies without a history of vaccination indicates either present or past infection with Covid19 based on the type of antibodies detected. Presence IgM could indicate a relatively recent infection, IgG antibodies may indicate later stage of illness or immunity to a past infection.

Absence of antibodies does not rule out infection. In acute infection PCR test remains the best choice.