Question

Explain the mechanisms of bacterial killing by neutrophil.

Answer 1

Neutrophils are phagocytic cells of the innate immune system. They respond to chemical signals given due to injury or infection like histamines, leukotrienes and interleukins and congregate at that site by chemotaxis within a few minutes. They eat foreign microbes, much like amoebas engulf their food particles. However, for this purpose, the microbe/antigen must be coated with antibodies existing in our blood.

Once the opsonized antigen (coated microbe) has been phagocytosed (engulfed), the antigen is lysed/digested by destroying it with hydrolytic enzymes and superoxide free radicals. To do so, the neutrophil activates an enzyme called NADPH oxidase (NADP = nicotinamide adenine dinucleotide phosphate, an energy giving biomolecule and reducing agent). But this enzyme requires oxygen (dioxygen) to produce the free radicals which are extremely damaging agents. For this purpose, the neutorphil also consumes oxygen in a burst, called 'respiratory burst'. Free radicals are produced according to the reaction:

NADPH + 2O₂ gives NADP⁺ + H⁺ + 2O₂⁻ where the dot represents the unpaired electron (2 electrons taken from NADPH are given to oxygen to produce the superoxide free radical)