A type of white blood cell that is an important part of the immune system and helps the body fight infection. When microorganisms, such as bacteria or viruses, enter the body, neutrophils are one of the first immune cells to respond. They travel to the site of infection, where they destroy the microorganisms by ingesting them and releasing enzymes that kill them. Neutrophils also boost the response of other immune cells. A neutrophil is a type of granulocyte and a type of phagocyte.