When you order a "complete blood count with differential" (CBC with diff), the laboratory examines the different types of white blood cells circulating in your blood.

Neutrophils are your body's first responders to bacterial infections. They make up more than half of all your white blood cells (typically 1.8–7.7 × 10⁹/L, or about 40-70% of white cells). When bacteria enter your body, neutrophils rush to the site, engulf the invaders, and destroy them.

High neutrophils (neutrophilia) often indicate a bacterial infection, inflammation, physical stress, or can be a response to certain medications like steroids. Low neutrophils (neutropenia) can occur with viral infections, certain medications, chemotherapy, or bone marrow problems, and may increase your risk of infections.