The SARS-CoV-2 virus, the cause of the current Covid-19 pendemia, can infect human respiratory epithelial cells through an interaction with the human ACE2 receptor or angiotensin converting enzyme receptor 2. SARS-CoV-2 has on its surface a protein called Spike or S protein. This type I transmembrane protein is present on the viral envelope in the form of a homotrimer. Each monomer consists of 2 subunits, S1 and S2. The S1 subunit contains a receptor binding domain (RBD) which binds to the ACE2 receptor on the host membrane. The S2 sub-unit is activated by the bond between S1 and the ACE2 receptor and contains the elements necessary for membrane fusion. The S protein thus plays an important role in the infection of SARS-CoV-2 as well as in the induction of neutralizing antibody and T cell responses and in protective immunity.