Leukocytes, or white blood cells, make up less than one percent of total blood volume. Their main function is immunological defense. There are two major categories of leukocytes, which are based on their structural characteristics: granulocytes contain membrane bound granules, while agranulocytes lack membrane bound granules. The total number of white blood cells is 4,500 to 10,000 per cubic millimeter of blood. A differential white blood cell count refers to the percentage of each of the five types of white blood cells.