Neutrophils are the most abundant type of white blood cell and serve as a first line of defense against infection and physiological stress. Mature neutrophils have a segmented nucleus, while immature forms—called “band” neutrophils—have a simpler, rod-shaped nucleus.

When the body responds to an acute stressor, particularly a bacterial infection, it releases more neutrophils into circulation, including these immature band forms. This increase, known as a “left shift,” reflects an active immune response. Band neutrophils are measured as part of a differential blood count, which evaluates the distribution of all white blood cell types.

In healthy individuals, band neutrophils typically make up 0–6% of total white blood cells. Elevated levels generally indicate infection or inflammation, though they can also rise with physical stress (e.g., intense exercise), certain medications, or, less commonly, bone marrow disorders.