Neutrophil.

A neutrophil is a type of white blood cell, an infection-fighting cell that plays a vital role in the body's immune system. Neutrophils are the most numerous white blood cells in the body, and the first to arrive at the site of an infection or injury. They are produced in the bone marrow and are short-lived, with life spans measured in hours. They travel through the body via the bloodstream and function as natural assassins, killing their target microorganisms in a number of ways. Abnormally high or low numbers of neutrophils in the body can result in several blood disorders and can be caused by genetic factors, immune disorders, or a response to medication or infection.