The first line of defense against infectious agents involves an active recruitment of phagocytes to the site of infection. Recruited cells include polymorhonuclear (PMN) leukocytes (i.e., neutrophils) and monocytes/macrophages, which function together as innate immunity sentinels (Underhill DM & Ozinsky A 2002; Stuart LM & Ezekowitz RA 2005; Flannagan RS et al. 2012). Dendritic cells are also present, serving as important players in antigen presentation for ensuing adaptive responses (Savina A & Amigorena S 2007). These cell types are able to bind and engulf invading microbes into a membrane-enclosed vacuole - the phagosome, in a process termed phagocytosis. Phagocytosis can be defined as the receptor-mediated engulfment of particles greater than 0.5 micron in diameter. It is initiated by the cross-linking of host cell membrane receptors following engagement with their cognate ligands on the target surface (Underhill DM & Ozinsky A 2002; Stuart LM & Ezekowitz RA 2005; Flannagan RS et al. 2012). When engulfed by phagocytes, microorganisms are exposed to a number of host defense microbicidal events within the resulting phagosome. These include the production of reactive oxygen and nitrogen species (ROS and RNS, RONS) by specialized enzymes (Fang FC et al. 2004; Kohchi C et al. 2009; Gostner JM et al. 2013; Vatansever F et al. 2013). NADPH oxidase (NOX) complex consume oxygen to produce superoxide radical anion (O2.-) and hydrogen peroxide (H2O2) (Robinson et al. 2004). Induced NO synthase (iNOS) is involved in the production of NO, which is the primary source of all RNS in biological systems (Evans TG et al. 1996). The phagocyte NADPH oxidase and iNOS are expressed in both PMN and mononuclear phagocytes and both cell types have the capacity for phagosomal burst activity. However, the magnitude of ROS generation in neutrophils far exceeds that observed in macrophages (VanderVen BC et al. 2009). Macrophages are thought to produce considerably more RNS than neutrophils (Fang FC et al. 2004; Nathan & Shiloh 2000).

The presence of RONS characterized by a relatively low reactivity, such as H2O2, O2˙− or NO, has no deleterious effect on biological environment (Attia SM 2010; Weidinger A & and Kozlov AV 2015). Their activity is controlled by endogenous antioxidants (both enzymatic and non-enzymatic) that are induced by oxidative stress. However the relatively low reactive species can initiate a cascade of reactions to generate more damaging “secondary” species such as hydroxyl radical (•OH), singlet oxygen or peroxinitrite (Robinson JM 2008; Fang FC et al. 2004). These "secondary" RONS are extremely toxic causing irreversible damage to all classes of biomolecules (Weidinger A & and Kozlov AV 2015; Fang FC et al. 2004; Kohchi C et al. 2009; Gostner JM et al. 2013; Vatansever F et al. 2013).

Although macrophages and neutrophils use similar mechanisms for the internalization of targets, there are differences in how they perform phagocytosis and in the final outcome of the process (Tapper H & Grinstein S 1997; Vierira OV et al. 2002). Once formed, the phagosome undergoes an extensive maturation process whereby it develops into a microbicidal organelle able to eliminate the invading pathogen. Maturation involves re-modeling both the membrane of the phagosome and its luminal contents (Vierira OV et al. 2002). In macrophages, phagosome formation and maturation follows a series of strictly coordinated membrane fission/fusion events between the phagosome and compartments of the endo/lysosomal network gradually transforming the nascent phagosome into a phagolysosome, a degradative organelle endowed with potent microbicidal properties (Zimmerli S et al. 1996; Vierira OV et al. 2002). Neutrophils instead contain a large number of preformed granules such as azurophilic and specific granules that can rapidly fuse with phagosomes delivering antimicrobial substances (Karlsson A & Dahlgren C 2002; Naucler C et al. 2002; Nordenfelt P and Tapper H 2011). Phagosomal pH dynamics may also contribute to the maturation process by regulating membrane traffic events. The microbicidal activity of macrophages is characterized by progressive acidification of the lumen (down to pH 4–5) by the proton pumping vATPase. A low pH is a prerequisite for optimal enzymatic activity of most late endosomal/lysosomal hydrolases reported in macrophages. Neutrophil phagosome pH regulation differs significantly from what is observed in macrophages (Nordenfelt P and Tapper H 2011; Winterbourn CC et al. 2016). The massive activation of the oxidative burst is thought to result in early alkalization of neutrophil phagosomes which is linked to proton consumption during the generation of hydrogen peroxide (Segal AW et al. 1981; Levine AP et al. 2015). Other studies showed that neutrophil phagosome maintained neutral pH values before the pH gradually decreased (Jankowski A et al. 2002). Neutrophil phagosomes also exhibited a high proton leak, which was initiated upon activation of the NADPH oxidase, and this activation counteracted phagosomal acidification (Jankowski A et al. 2002).

The Reactome module describes ROS and RNS production by phagocytic cells. The module includes cell-type specific events, for example, myeloperoxidase (MPO)-mediated production of hypochlorous acid in neutrophils. It also highlights differences between phagosomal pH dynamics in neutrophils and macrophages. The module describes microbicidal activity of selective RONS such as hydroxyl radical or peroxynitrite. However, detection of any of these species in the phagosomal environment is subject to many uncertainties (Nüsse O 2011; Erard M et al. 2018). The mechanisms by which reactive oxygen/nitrogen species kill pathogens in phagocytic immune cells are still not fully understood.