Illustration of an immune response to infection with the new coronavirus SARS-CoV-2 (one at top left). Receptors on a B cell's (red) surface recognise and bind to proteins (antigens) on the viral surface and ingest them before presenting them on MHC (major histocompatibility complex) class II molecules on the cell surface (lower centre). T helper cells (turquoise) recognise the antigen attached to the MHC molecule and secrete cytokines (small spheres) that activate other immune cells to mount an immune response against the virus. SARS-CoV-2 emerged in Wuhan, China, in December 2019, and causes a mild respiratory illness (covid-19) that can develop into pneumonia and be fatal in some cases.