CD23, is expressed on a variety of haematopoietic cell types and displays pleiotropic activities in vitro. Here Jean-Yves Bonnefoy and collegues discuss a novel interaction between CD23 and the alpha chains of the beta(2) integrins, CD11b and CD11c, that leads to a proinflammatory pattern of macrophage activation. They describe how neutralizing antibodies to CD23 can decrease the severity of murine arthritis.