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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.
Here are the six main types of WBCs and the average percentage of each type in the blood:
Neutrophils - 58 percent
Eosinophils - 2 percent
Basophils - 1 percent
Bands - 3 percent
Monocytes - 4 percent
Lymphocytes - 4 percent
Most WBCs (neutrophils, eosinophils, basophils and monocytes) are formed in the bone marrow. Neutrophils, eosinophils and basophils are also called granulocytes because they have granules in their cells that contain digestive enzymes. Basophils have purple granules, eosinophils have orange-red granules and neutrophils have a faint blue-pink color. When a granulocyte is released into the blood, it stays there for an average of four to eight hours and then goes into the tissues of the body, where it lasts for an average of four to five days. During a severe infection, these times are often shorter.