The blood has several types of white blood cells including neutrophils, bands, eosinophils, basophils, monocytes and lymphocytes. Each fights infection in a different way.
Neutrophils, for example, are one of the body's main defenses against bacteria. Neutrophils kill bacteria by ingesting them. Neutrophils can ingest five to 20 bacteria in their lifetime.
Another type of white blood cell is called bands. Bands are immature neutrophils. When a bacterial infection is present, an increase of both neutrophils and bands are seen.
The eosinophil type of white blood cell kills parasites and has a role in allergic reactions.
Although basophils are not well understood, they also function in allergic reactions. Basophils release histamine (which makes blood vessels leak and attracts white blood cells) and heparin (which prevents clotting in an infected area so that the white blood cells can reach the bacteria).
Monocytes enter the tissue itself, where they become larger. There they can ingest bacteria throughout the body. These cells also destroy damaged and dead cells.
Neutrophils and monocytes use many mechanisms to get to and kill invading organisms. Both can squeeze through openings in blood vessels. They are attracted to chemicals produced by the immune system or by bacteria. Then, they migrate toward areas where the concentration of these chemicals is high. They kill bacteria by completely surrounding the bacteria and digesting them with enzymes.
Lymphocytes are complex cells used to direct the body's immune system. White blood cells called lymphocytes come in two types, T lymphocytes (called T cells) and B Lymphocytes. About 75 percent of lymphocytes are T cells.