Fig. 1. Culture of neutrophils without lipopolysaccharide under normoxia-normoxia, hypoxia-normoxia, and hypoxia-hyperoxia. Normoxia group: No insult was given to this group. Polymorphonuclear neutrophils (PMN) was cultured and incubated at 37℃ in a 5% CO₂ incubator in room air. Normoxia group was control group. Hypoxia-normoxia group: An initial hypoxia insult was administered for 2 hours; then normoxic insult and 2 hours of culture at 37℃ in 5% CO₂ incubator followed by incubation in room air for 14 hours. Hypoxia-hyperoxia group: An initial hypoxia insult for 2 hours was given, then hyperoxic insult and 2 hours of culture at 37℃, followed by incubation in room air for 14 hours. Hydrogen peroxide (H₂O₂) and Toll-like receptor 4 (TLR₄) were measured after 4 hours of PMN culture. Apoptosis was measured after 18 hours, including period of first and second insults and culture for 14 hours in room air.