Neutrophils, which are produced in the bone marrow and circulate in the blood, are a type of white blood cell. Neutrophils, are abundant and make up about 50% to 75% of white blood cells. Neutrophils respond to infection and attack bacteria and other foreign invaders directly.
Neutrophils are the first type of immune cell to respond to and arrive at the site of infection, often within an hour. Neutrophils will respond to infection inside the body, but also on the surface, as in the case of skin infections. Pus, which is one visible sign of a skin infection, contains mainly dead neutrophils, bacteria, and cells. Pus can form internally, as well.
The results of a common blood test, called an absolute neutrophil count (ANC), are routinely checked during cancer treatment to determine how the immune system is responding to treatment. When ANC is low, this is called neutropenia. If ANC drops below about 500 cells per microliter, the risk of infection increases and your doctor may prescribe medications to bring ANC back into a normal range and temporarily offer antibiotics to prevent serious infections.
Neutrophils are part of the innate immune system, which means that they can "non-specifically" destroy any invaders that they encounter in the body, such as bacteria and parasites. Non-specifically means that neutrophils do not have to recognize the invader specifically, but instead simply recognize the invader as something that should not be present in the body and should be destroyed.