How we vary in our genetic code has a critical impact on events throughout our lives that our influenced by our immune system, especially susceptibility to infection. It is increasingly clear that genetic changes can influence individual patient’s ability to respond to cancer immunotherapy – in terms of both cancer shrinking, but also the development of side effects from treatment – so called ‘immune related Adverse Events’. Patients who develop irAEs from immunotherapy tend to have better clinical outcomes – but this is not always the case, and some patients have excellent clinical outcomes without getting side effects. Nonetheless, the links between patient genetics, there immune responses to immunotherapy, development of irAEs and response to the tumour are very strong. By studying these we are developing a much better picture of how immunotherapy works and hope that we will be able to take these findings back to the clinic to better target treatments.