Professor Heath has uncovered basic rules on how an immune response to pathogens is initiated. He studied interactions between dendritic cells, which collect, process and present antigens, and T cells which respond and destroy infected cells. He found that particular types of dendritic cells are specialised for activating killer T cells, but these dendritic cells must first be “licensed” or activated by helper T cells. He has also shown how the balance is changed in a non-infected individual, so the same system avoids responding to the bodies’ own tissues.