MHC molecules are normal components of healthy cells.
They were known to differ among individuals and to cause rejection of organ transplants and they are therefore sometimes called transplantation antigens. It came as a surprise that they were also involved in recognition of infected cells. Doherty and Zinkernagel presented two main theoretical models to explain their observations. These models have inspired immunologists and set the stage for research on cell-mediated immunity for at least two decades. Through important discoveries made later by other scientists, we are now getting a clearer picture of the scenario.