what is a spike protein

A spike protein is a protein that forms a large structure known as a spike or peplomer projecting from the surface of an enveloped virus. The spike protein is found on the surface of SARS-CoV-2, the virus that causes COVID-19, and is responsible for mediating viral entry as the first step in viral infection. The spike protein is highly antigenic and accounts for most antibodies produced by the immune system in response to the virus. COVID-19 vaccines use the spike protein to trigger an immune response in the body. Spike proteins are usually glycoproteins that form dimers or trimers and are membrane proteins with typically large external ectodomains, a single transmembrane domain that anchors the protein in the viral envelope, and a short tail in the interior of the virion.