All white blood cells are known officially as Leukocytes . White blood cells behave very much like amoeba in their movements and are able to engulf other cells and bacteria. Many white blood cells cannot divide and reproduce on their own, but instead have a factory somewhere in the body that produces them. That factory is the bone marrow. Granulocytes get their name because they contain granules, and these granules contain different chemicals depending on the type of cell.
Monocytes evolve into macrophages.
All white blood cells start in bone marrow as stem cells . Stem cells are generic cells that can form into the many different types of leukocytes as they mature. For example, you can take a mouse, irradiate it to kill off its bone marrow's ability to produce new blood cells, and then inject stem cells into the mouse's blood stream. The stem cells will divide and differentiate into all different types of white blood cells. The stem cells find their way, almost magically, into the marrow and make their home there.