Viruses infect host cells and reproduce inside them. Killer T-cells destroy those infected cells so that the viruses can't reproduce. Zinkernagel and Doherty discovered that, in order for killer T-cells to recognize infected cells, they had to recognize two molecules on the surface of the cell—not only the virus antigen, but also a molecule of the major histocompatibility complex (MHC). This recognition was done by a T-cell receptor on the surface of the T-cell. The MHC was previously identified as being responsible for the rejection of incompatible tissues during transplantation. Zinkernagel and Doherty discovered that the MHC was responsible for the body fighting meningitis viruses too.