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Why can immature white blood cells appear in the blood during a severe infection?

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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.

During a severe infection, the body’s demand for white blood cells—specifically granulocytes like neutrophils—increases significantly to combat pathogens.12

When the bone marrow is stimulated to produce these cells at a rapid rate, it may be unable to keep up with the body's urgent requirements for mature cells. Consequently, the bone marrow releases younger, immature forms of these cells, such as myelocytes, into the bloodstream to help meet the demand. This phenomenon is often associated with leukocytosis, an abnormal increase in the total number of white blood cells. As the infection subsides and the demand for new cells decreases, the bone marrow is able to return to its normal production cycle, and these immature forms disappear from circulation.21