Illustration of the m396 antibody against the SARS (severe acute respiratory syndrome) coronavirus. This antibody targets the receptor binding domain of the SARS spike protein. Antibodies are proteins of the immune system that bind to specific antigens, such as a viral protein, and either mark the pathogen for destruction by other immune cells, or destroy it themselves. The SARS spike protein binds to angiotensin-converting enzyme 2 (ACE2) receptors on human cells, facilitating the virus's entry to the cell. The SARS virus caused a global outbreak of a pneumonia-like disease in 2003.