Under normal conditions, immunoglobulin G (IgG) antibodies are anti-inflammatory, yet during infection, they can protect the body by recruiting inflammatory cells or pathways. Kaneko et al. now show that IgG can alternate between these states by modifying the sialylation of polysaccharides on the nonvariable part of the IgG that is responsible for binding and activating receptors on immune cells. These receptors bind sialylated forms of IgG less strongly, which signals the cell to load its surface with inhibitory proteins. However, under situations of immune challenge, the sialic acid residues are lost, which allows IgG to bind more tightly and so turn up inflammatory signaling. By switching activity in this way, IgG antibodies may restrict their pro-inflammatory effects to periods of infection and so limit the potential for inadvertent harm resulting from their activity.