Neutrophils are a kind of white blood cell. Filled with enzymes, they help phagocytic cells kill and digest microorganisms that they have engulfed. Neutrophils are the body's main immune response to assaults on the body by bacteria and fungi. (They seem to have no effect on viruses and thus are not put into play when dealing with viral infections such as the common cold.) When an invader enters the bloodstream, tissue, etcetera, the neutrophils are brought to the scene by a process called chemotaxis.
In chemotaxis, the neutrophils recognize chemicals that are produced by bacteria and leave the blood vessels and go into the tissues following the trail of chemicals. When they find a bacteria cell, they completely surround it and digest it (basically tear it appart with chemicals). This also kills the neutrophil which the body expells. Dead neutrophils and digested bacteria are the main component in pus that forms around infected cuts.
In addition to killing bacteria by phagocytosis (engulfing), the neutrophils also can release a jumble of fibers in the form of a net called a neutrophil extracellular trap (NET). This kills microbes outside of the cell when the neutrophils release proteins in granules. Neutrophils also release superoxide. This is converted into hypochlorous acid (also known as chlorine bleach), which may also play a part in killing microbes.
Even when not engaged in ridding the body of invaders, the neutrophils have a very short life, lasting no more than a few weeks. In a normal human body, there are many constantly floating around in the bloodstream, looking for an infection. When a body has an insufficient number of neutrophils, it is called neutropenia. This can be a side effect of chemotherapy, and may also result from leukemia or anemia. Insufficient numbers of these important cells can hinder the body's immune response. Malfunctioning neutrophils can also cause trouble by actually damaging human tissues. Elevated levels of neutrophils may occur when the body is fighting a flu or other infection.
Neutrophils are often called the body's first line of defense, after about 48 hours, macrophages will begin to come into play. In the average body, there is one neutrophil for every one-thousand red blood cells. The relatively small neutrophil gets its energy from fermentation. Neutrophils are made by stem cells which go through terminal differentiation, which means they select to become a specified cell. Most of the body's neutrophils remain in the bone marrow. If the body is infected, many of them will be released at once to fight the infection.1 Neutrophils that complete their lifespan within the bone marrow without being used are generally consumed by macrophages. Others, when they die, disintegrate and provide food for other cells.