White Blood Cells (Leukocytes):

These blood cells are colorless, as they do not contain pigments. They are not confined to the bloodstream, as they also migrate out into the tissue fluid.

Amount of White Blood Cells:

There are 1 or 2 leukocytes for every 1000 RBCs. One cubic millimeter of blood contains 7000 to 8000 of them. They are much larger (two to three times) than the red blood cells.

The average life span of a white blood cell:

They have a life span of months or even years, but this depends on the body's needs.