Several receptors of phagocytes (activated neutrophils and macrophages) recognize microbes, which include, TLRs, G-protein coupled receptors, Fc and C3 receptors etc.
Immediately after binding to foreign particles, phagocytes increase their oxygen consumption to about 100 fold and convert molecular oxygen into reactive oxygen intermediates (ROIs), which are oxidizing agents and destroy microbes. This metabolic pathway is also known as hexose-monophosphate shunt.
The primary free radical generating system is the phagocyte oxidase system. When phagocyte bind with foreign particle, the cell surface enzyme, NADPH (Nicotinamide-adenine dinucleotide phosphate) -oxidase is activated and reduced to NADP with release of electrons. These electrons are accepted by molecular oxygen and form ROIs such as super oxide radicals where NADP acts as cofactor.
Super oxide is dismuted enzymatically to form hydrogen peroxide (H2O2). Myeloperoxidase enzyme of the phagocyte catalyzes the reaction between hydrogen peroxide and intracellular halide ions to form hypohalides that are toxic to the bacteria. The process by which ROIs are produced is called the respiratory burst.
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