White blood cells, also called leukocytes, are the cells responsible for fighting infections by protecting the body from foreign invaders. Unlike red blood cells, white blood cells do not contain hemoglobin, therefore they are clear-colored. New white blood cells are continually produced in the bone marrow to replace old ones, which have a lifespan of only a few days. There are five different types of white blood cells, each with a unique function.
Neutrophils
Neutrophils are the most plentiful, making up 55 to 70 percent of the total white blood cell count. Neutrophils are able to receive chemical signals that can differentiate foreign invaders such as bacteria or viruses. The neutrophils then follow the chemicals until they find the invader, quickly engulfing it until it dissolves. Neutrophils only live a few days, and the dead cells become the pus that is excreted from the infection.
Eosinophils
The normal concentration of eosinophils in the blood is very low, only 1 to 3 percent, according to the American Partnership for Eosinophilic Disorders. Just as the other white blood cells, eosinophils are produced in the bone marrow; however, their production is stimulated by the presence of cytokines, specific proteins secreted during an inflammatory response.
Eosinophils attack foreign invaders, with a special affinity for parasites. Because parasites are too big to be engulfed by immune cells, eosinophils secrete powerful substances that can kill the parasites. Eosinophils are also produced in response to an allergic reaction, especially to food allergens. This explains why eosinophils are often found in higher concentrations in the gastrointestinal system.
Basophils
Basophils are very small white blood cells that make up less than 1 percent of the total white blood cell count. Although it is known that basophils are present during an immune response, their specific mechanism of action remains unknown.
Monocytes
Monocytes are the largest of the white blood cells and comprise 5 to 8 percent of the total white blood cell count. Although large, they are able to squeeze between the cells in the walls of blood vessels and enter tissues. Once in the tissues, the monocytes are called macrophages.
A macrophage, which means "big eater," does just that--it consumes bacterial cells. Macrophages are also able to secrete chemicals to attract other white blood cells like neutrophils to the site of infection.
Lymphocytes
There are several different types of lymphocytes, including B lymphocytes and T lymphocytes. B lymphocytes are white blood cells that produce antibodies against toxins secreted by foreign invaders. T lymphocytes, known as inflammatory T cells, send signals to macrophages and neutrophils, calling them to sites of infection. Cytotoxic T lymphocytes specifically target virus and tumor cells. Helper T lymphocytes help the B lymphocytes to produce antibodies.
