Figure 5.

Formation of ROS by phagosomes. NADPH oxidase (Nox) generates superoxide which cannot penetrate the cytoplasmic membranes of the engulfed bacteria. It is believed that either superoxide or its protonated form injures extracytoplasmic targets. Additionally, membrane-permeable H₂O₂ is generated through dismutation. Calculations suggested that the levels of ${\text{O}}_2^{-}$, HO₂• and H₂O₂ in isolated macrophages range from 10–50 μM, 0.1–4 μM and 1–4 μM, respectively, depending upon phagosomal pH. Modeling predicts a similar H₂O₂ concentration inside neutrophils. The H₂O₂ levels would rise, however, if it accumulates it the surrounding tissue.