IFNG and infection: In combination with the cytokine interleukin (IL)-12 secreted by the professional APCs and necessary co-stimulatory molecules (‘signal two’ and ‘signal three’), TH0 cells differentiate into TH1 effector cells that proliferate and produce massive quantities of pro-inflammatory cytokine interferon gamma (IFN-γ), as well as IL-2 and tumour necrosis factor (TNF), triggering the influx of inflammatory cells and the complement system into the site of infection, or into TH1 memory cells.