White cells are irregular in shape and size, but generally are larger than the red cells.
They differ from the red cells in that each white cell contains a nucleus. Adult humans have between 4,000 and 11,000 leukocytes per cubic millimeter of blood. In infants, the number is essentially doubled. There is a ratio of roughly one white cell for every 700 red cells. In leukemia, there is an uncontrolled increase in the number of leukocytes.