Illustration of antibodies (blue) responding to a severe acute respiratory syndrome (SARS) virus infection. SARS is a viral respiratory illness caused by a coronavirus, called SARS-associated coronavirus (SARS-CoV). SARS coronavirus is an enveloped RNA (ribonucleic acid) virus. Within the membrane are spike proteins (yellow and red). Antibodies bind to specific antigens, for instance viral proteins, marking them for destruction by phagocyte immune cells.