Neutrophil

The neutrophil is the most abundant white blood cell: between 50 and 70% of the white cells circulating in the blood. It is the body's first line of defense. Under the microscope you recognize it by its nucleus, which instead of being a single round mass is split into several lobes joined by thin strands. That is why it is also called a polymorphonuclear cell, a name that means many-shaped nucleus.

It kills in a set sequence of steps. It recognizes foreign antigens, markers that do not belong to the body, with receptors on its membrane. Then it swallows the bacterium and seals it inside a phagolysosome, an internal sac loaded with enzymes. It traps the invader in a sac and releases the substances that destroy it right there. Part of that is the respiratory burst, a discharge that generates reactive oxygen species, highly aggressive oxygen molecules.

They are the cells that show up in the greatest numbers at the site of an injury or an infection. And they do not last long: they live only a few days and they die once they are activated in the tissue, where macrophages, another kind of defense cell, then clear them away.