The idea of using patients’ own immune cells to fight cancer is over a hundred years old. In the late 19th century, an American doctor called William Coley attempted to treat cancer by stimulating the immune system with dangerous bacteria, arguably the first example of what is now known as immunotherapy. His procedure appeared to shrink some patients’ tumours, but was criticised as unsafe and largely forgotten about. Chemotherapy and radiotherapy became the focus of cancer research and the standard tumour treatment for the rest of the 20th century.