Figure 3.

Neutrophils express several groups of receptors that can induce the formation and generation of reactive oxygen species. Activation of integrin and Fc receptors leads to complex intracellular signal transduction pathways that can robustly activate the NADPH oxidase complex (solid black arrows). Some members of G-protein-coupled receptors (GPCRs) family, specifically formyl receptors, can directly activate NADPH oxidase, although to a lesser extent than to what has been observed in integrin and Fc receptors (dotted black arrow). Ligand binding to TLRs, TNFRs, and some members of GPCRs can transform the neutrophils into an “primed” state, whereby the NADPH oxidase is more susceptible to activation by a secondary stimulus (purple dotted arrows). This is presumably another level of regulation to ensure that reactive oxygen species are produced at the right time and place that is only during an active infection.