The increasing globalisation of infectious disease is a major challenge to human health. The MRC Human Immunology Unit is a key player in international efforts to combat this threat, and in research into other diseases involving the immune system. The immune system is crucial to human health. Our ability to identify and destroy invading pathogens involves complex networks of interacting cells and molecules. Understanding precisely how the system works at the cellular, genetic and molecular levels will help in the development of new therapies for diseases such as AIDS, avian flu, multiple sclerosis, arthritis and eczema.