PM2.5 counteracts with IL-4-induced M2 polarization partially through mammalian target of rapamycin (mTOR) pathway, whereas in ovalbumin (OVA)-induced asthma, PM2.5 aggravates allergic inflammation by activating M2 macrophages and subsequent Th2-related immune responses [21,30]. The gene discussed is IL4; the disease is asthma.