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Why do bacterial infections often increase the number of neutrophils in the blood?

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Illustration of a blood vessel with a cutaway view showing red blood cells, white blood cells, platelets, and plasma, each labeled to highlight the main components of blood. — white blood cell Blood is made up of multiple components, including red blood cells, white blood cells, platelets, and water-rich plasma.

Bacterial infections trigger an increase in the number of neutrophils in the blood—a condition known as neutrophilia—primarily as a protective response to combat microbial invasion.1

Because neutrophils are short-lived cells that are rapidly depleted while engulfing and destroying bacteria, the body maintains a large reserve of them in the bone marrow to be mobilized during infection or inflammation.1

The process is regulated by signaling molecules called colony-stimulating factors (CSFs). When a bacterial infection occurs, specific bacterial components accelerate the production of granulocyte CSF. This protein acts as a signal to the bone marrow, stimulating the division and differentiation of precursor cells to increase the production of phagocytic granulocytes, ensuring the body can effectively address the infection.2