White blood cells are the body’s sentries, serving as the backbone of the immune system. White blood cells are found throughout the body, in both the blood and the lymphatic system. White blood cells have a density of about 4-11 billion per liter of blood. The scientific name for a white blood cell is leukocyte, simply meaning “white cell.”

White blood cells get their name from their color – white. This color was first discovered when blood was put into high-speed centrifuges and separated into its components. In healthy individuals, white blood cells make up about 1% of the blood. In sick individuals, this percentage increases substantially. The ratio of white blood cells in the blood can be used as a diagnostic of illness.

There are several different types of white blood cells, all with subtly varying functions. Some of the most common are neutrophils (65%), lymphocytes (25%), monocytes (6%), eosinophils (4%), and basophils (1%). White blood cells originate from stem cells in bone marrow. Scientists are working on ways to use stem cells to mass-produce white blood cells at will, which could be used for immune-boosting therapies.

Neutrophils are the most common and the first line of defense against bacterial and fungal infections. Pus consists of large numbers of dead neutrophils. Neutrophils, like most white blood cells, engage in phagocytosis, that is, the consumption and digestion of bacteria or other foreign materials. Neutrophils can detect if a cell is part of the body or foreign by checking the molecules on its surface. Neutrophils function as suicide bombers – they cannot replenish the lysosomes used to digest microbes, so after killing a few bacteria or fungi, they simply die.

Lymphocytes are the second most common type of white blood cell. There are a variety of different lymphocytes. Their functions include the production of antibodies — protein tags that help the body identify invaders — immune system coordination, and destruction of body cells which have become contaminated with foreign material.