People with neutropenia have an unusually low number of cells called neutrophils. Neutrophils are cells in your immune system that attack bacteria and other organisms when they invade your body.Neutrophils are a type of white blood cell. Your bone marrow creates these cells.eople with neutropenia have an unusually low number of cells called neutrophils. Neutrophils are cells in your immune system that attack bacteria and other organisms when they invade your body.