T cells protect us from infection. In our daily lives, we’re constantly exposed to pathogens, such as bacteria, viruses and fungi. Without T cells, every exposure could be life-threatening. T cells can wipe out infected or cancerous cells. There are two major types of T cells: the helper T cell and the killer T cell. As the names suggest helper T cells ‘help’ other cells of the immune system, whilst killer T cells kill virally infected cells and tumours.