Activation of the NADPH oxidase enzyme complex results in the production of the oxygen free radical, superoxide anion (O2.-). Superoxide anion is critical for host defense against fungal and bacterial pathogens and efficient immune responses; however, uncontrolled monocyte-derived O2.- may contribute to chronic inflammation and tissue injury. We have previously identified several pathways that regulate the activity of NADPH oxidase in human monocytes; however, the receptor(s) responsible for the activation of NADPH oxidase in primary human monocytes have not yet been determined. This...