Spike protein: A spike protein is a protein structure that protrudes from the surface of a virus. The structure of the protein is shaped such that it resembles a spike. These spikes are common to coronaviruses, such as the SARS-CoV-2/COVID-19 coronavirus. Spike proteins play a significant role in the ability of the virus to enter a cell. Spike proteins help to attach the virus to specific receptors on a cell surface, and then once attached, allow the viral membrane to fuse with the cell membrane and thereby infecting the cell with the viral payload. Because spike proteins on viruses are recognized by the human body's immune system, these proteins are a major source of research for the development of vaccines against coronaviruses.