White blood cells (WBCs), or leukocytes, are a part of the immune system and help our bodies fight infection. They circulate in the blood so that they can be transported to an area where an infection has developed. In a normal adult body there are 4,000 to 10,000 (average 7,000) WBCs per microliter of blood. When the number of WBCs in your blood increases, this is a sign of an infection somewhere in your body. Tere are the six main types of WBCs: Neutrophils, Eosinophils, Basophils, Bands, Monocytes, Lymphocytes.

• Neutrophils are the one of the body’s main defenses against bacteria. They kill bacteria by actually ingesting them (this is called phagocytosis). They can phagocytize five to 20 bacteria in their lifetime. 
• Bands are immature neutrophils that are seen in the blood. When a bacterial infection is present, an increase of neutrophils and bands are seen.
• Eosinophils kill parasites and have a role in allergic reactions. 
• Basophils are not well understood, but they function in allergic reactions. They release histamine (which causes blood vessels to leak and attracts WBCs) and heparin (which prevents clotting in the infected area so that the WBCs can reach the bacteria).
• Monocytes enter the tissue, where they become larger and turn into macrophages. There they can phagocytize bacteria (up to 100 in their lifetime) throughout the body. These cells also destroy old, damaged and dead cells in the body. Macrophages are found in the liver, spleen, lungs, lymph nodes, skin and intestine. The system of macrophages scattered throughout the body is called the reticuloendothelial system. 

Neutrophils and monocytes use several mechanisms to get to and kill invading organisms. They can squeeze through openings in blood vessels by a process called diapedesis. They move around using ameboid motion. They are attracted to certain chemicals produced by the immune system or by bacteria and migrate toward areas of higher concentrations of these chemicals. This is called chemotaxis. They kill bacteria by a process called phagocytosis, in which they completely surround the bacteria and digest them with digestive enzymes.