Neutrophils are the most common type of white blood cell, comprising about 50-70% of all white blood cells. They are phagocytic, meaning that they can ingest other cells, though they do not survive the act. Neutrophils are the first immune cells to arrive at a site of infection, through a process known as chemotaxis.

Though neutrophils are short lived, with a half-life of four to ten hours when not activated and immediate death upon ingesting a pathogen, they are plentiful and responsible for the bulk of an immune response. They are the main component of pus and responsible for its whitish color. Neutrophils are present in the bloodstream until signaled to a site of infection by chemical cues in the body. They are fast acting, arriving at the site of infection within an hour.

Before ingesting invasive bacteria, neutrophils can release a net of fibers called a neutrophil extracellular trap (NET), which serves to trap and kill microbes outside of the cell. When neutrophils ingest microbes, they release a number of proteins in primary, secondary, and tertiary granules that help kill the bacteria. They also release superoxide, which becomes converted into hypochlorous acid, or chlorine bleach, which is theorized to play a part in killing microbes as well.

A deficiency of neutrophils is called neutropenia and may be congenital or acquired, for example in certain kinds of anemia and leukemia, or as a side effect of chemotherapy. Since neutrophils are such an important part of the immune response, a lowered neutrophil count results in a compromised immune system.

Neutrophils may also malfunction, causing more harm to the body than they prevent. In alpha 1-antitrypsin deficiency, for example, inflammation, part of a normal immune response, leads to tissue damage. In Familial Mediterranean fever, the immune response is also so acute and prolonged that it can lead to a number of dangerous complications.