For the past 15 years or so, a class of drugs called immune checkpoint inhibitors have been used to treat melanoma – the most dangerous kind of skin cancer . For many patients, they produce remarkable results. For others, they do nothing. We still don't really know why. But in new research published in the Journal of Experimental Medicine , we observed immune cells called macrophages attacking melanoma cells in real time – which may offer clues about how we can make those therapies work for all