Low levels of antibodies inside of a tumour can in fact stimulate tumour growth, by causing inflammation. This switches on pathways such as NF-κB and STAT3. However, it has long been thought that the immune system, if properly engaged, could be used to eliminate cancer cells. Here, the authors show that antibodies targeting Neu5Gc (which accumulates in human tumours) have a dose-dependent affect on tumour mass. In particular, low levels promote tumour growth, but over a surprisingly narrow dose range, the antibody suddenly begins to inhibit tumour growth.