Antibodies are proteins produced by the immune system that bind to antigens to neutralise them or mark them for destruction by other wings of the immune system. The spike protein on SARS-CoV-2 is the protein antigen that allows the virus to attach to cells in our body to initiate an infection.
If our immune system has antibodies against the S antigen on the virus – resulting from exposure to SARS-CoV-2 or from vaccination – they can bind to it and help prevent us from becoming infected. Other antibodies against SARS-CoV-2 have other roles, for example in recruiting other wings if the immine system.
With a SARS-CoV-2 infection, antibodies called IgM appear 7-10 days after infection and wane as the infection does.
IgG arrive after this but remain detectable longer—term; these alert the immune system to reinfection.
IgG against the S protein of the virus are a cornerstone of long-term immunity and there is a diagnostic cassette for the AFS-1000 to measure them in serum using a fingerprick blood test.