In this sequence of microscope images, an amoeboid human neutrophil senses, moves toward and ingests an ovoid yeast. The indicator dye nitroblue tetrazolium (NBT) demonstrates that the white cell is using its lethal oxidative ability to kill the yeast. These images are from black and white time-lapse video and have been color enhanced to show the degree and location of the oxidative burst.

The "respiratory burst" describes a metabolic pathway, dormant in resting cells, whose function is to produce a group of highly reactive microbicidal agents by the partial reduction of oxygen. Killing of invading microbes is accomplished through the action of ozidizing agents provided by the respiratory burst. Bacterial killing involves multiple mechanisms set into motion by two cellular events: degranulation and the initiation of the "respiratory burst".

The attachment of microorganisms to phagocyte membrane initiates the process of phagocytosis (phagosome formation) and causes the activation of the respiratory burst (hexose monophosphate shunt) which results in the production of superoxide anion, singlet oxygen, hydroxyl ion and hydrogen peroxide. These molecules are microbicidal and cause killing of organisms in the phagosome.

A phagosome, soon after its formation, fuses with granules (lysosomes) to form a phago-lysosome. As mentioned earlier, lysosomes contain a variety of anti-microbial substances and phago-lysosome fusion results in the exposure of microorganisms to these substances and their destruction.
Fusion of phagosome with primary granules exposes its content to myeloperoxidase which catalyzes production of toxic oxidants, halogenation of bacterial proteins and microbial death.

It should be apparent from the preceding discussion that there are three pathways of intracellular killing of microbes. (1) by lysosomal antibacterial substances (lactoferrin, cationic proteins, lysozyme, defensins, proteases, etc.) without the requirement of respiratory burst (oxygen- independent killing). (2) by products of the respiratory burst (super-oxide, singlet oxygen, hydroxyl radical, hydrogen peroxide, etc.) without the need for myeloperoxidase (oxygen-dependent, myeloperoxidase- independent killing). (3) by hydrogen peroxide metabolites halogenation of bacterial proteins catalyzed by myeloperoxidase (oxygen-dependent, myeloperoxidase-dependent killing: Figure 5B). A defect in any of these pathways, for example, due to absence of NADPH oxidase (cytochrome b558: p91-, p22, 947- & p61-phox), myeloperoxidase, etc. may predispose the individual to increased susceptibility to pyogenic infections. Neutrophils also contain catalase and glutathione (GS) which detoxify excess H2O2. GS, in its reduced form (GSH), also recycles NADP to NADPH. Interaction of phagocytic cells with certain humoral factors (e.g. interferons, TNF, C5a, IL-2, etc.) can increase their phagocytic function, respiratory burst and intra-cellular killing. Some cytokines can also induce phagocytic cells, particularly macrophages, to produce nitric oxide (NO), which is toxic to microorganisms and malignant cells. Respiratory burst: Oxygen dependent myeloperoxidase reactions. Respiratory burst: Oxygen dependent myeloperoxidase dependent reactions.