Spike proteins are specific proteins found on the surface of certain viruses, including the SARS-CoV-2 virus that causes COVID-19. These proteins play a crucial role in how the virus enters human cells. They bind to receptors on the surface of cells, allowing the virus to attach and gain entry, which is essential for infection. In the context of vaccines, such as the Pfizer-BioNTech and Moderna vaccines, spike proteins are used to train the immune system. The vaccines introduce a harmless piece of the spike protein, prompting the body to recognize and respond to it, thereby preparing the immune system to fight the actual virus if encountered.