TLR9 and macrophage activation syndrome: The third and final type, known as CpG-induced MAS, involves relentless antigenic stimulation of the NLRP3 inflammasome, through activation of toll-like receptor 9 (TLR9), which recognises DNA rich in unmethylated CpG–DNA motifs from bacterial or viral DNA, again driving elevated fIL-18 levels, and subsequent elevated IFN-gamma levels [34].