Increasing evidence points to a central role for immune dysregulation in autism spectrum disorder (ASD). Several ASD risk genes encode components of the immune system and many maternal immune system-related risk factors — including autoimmunity, infection and fetal reactive antibodies — are associated with ASD. In addition, there is evidence of ongoing immune dysregulation in individuals with ASD and animal models of this disorder. Recently, several molecular signalling pathways have been identified that link immune activation to ASD phenotypes, including pathways downstream of cytokines, hepatocyte growth factor receptor (MET), MHCI molecules, microglia and complement factors. These findings indicate that the immune system is a point of convergence for various ASD-related genetic and environmental risk factors.