Leukocytes AnatomyCredit: http://www.alliedhealthworld.com/blog/?p=236Leukopoiesis is the term used to describe the production of white blood cells. The process is signaled by certain chemical messengers. The chemical messengers act as either hormones or paracrines. 
Birth of LeukocytesBirth of LeukocytesCredit: http://www.faqs.org/health/Body-by-Design-V1/The-Cardiovascular-System-Design-parts-of-the-cardiovascular-system.html
The chemical messengers that trigger the production of leukocytes are glycoproteins that fall under the families of hematopoietic factors, namely the interleukins and the CSF or colony-stimulating factors. The mature WBCs and the supporting cells of bone marrow release the hematopoietic factors. These hematopoietic factors not only prompts the precursors of white blood cells to divide and mature but it also enhances the potency to protect of mature leukocytes.
A number of hematopoietic hormones are being used clinically 
in order to stimulate bone marrows of cancer patients who are receiving chemotherapy and those who received bone marrow transplants. 
Pathways of Leukocyte Differentiation Pathways of Leukocyte DifferentiationCredit: http://www.london-research-institute.org.uk/news-and-events/all-news/highlighted-paper-activated-leukocyte-cell-adhesion-molecule-alcam
In the early stages, a division between the lymphoid stem cells and the myeloid stem cells occur. The lymphoid stem cells are the ones that produce lymphocytes while myeloid stem cells are responsible for producing all other formed elements. In the formation of granulocyte, myeloblasts first appear and with enough accumulated lysosomes they become promyelocytes. As the promyelocytes continue to mature, distinctive granules appear and then they become myelocytes. When they become myelocytes, cell division stops. The myelocytes then continue to mature as just before they leave the bone marrow as granulocytes, their nuclei constricts, beginning the process of nuclear segmentation. 
There is an even greater number of stored granulocyte compared to the number of granulocytes circulating in the bloodstream. Most of the granulocytes die fighting with invading microorganism; hence the body tends to create more. The normal ratio of granulocytes to erythrocytes produced is 3:1.
Leukocyte Disorders
 Leukocyte DisordersCredit: http://www.docstoc.com/docs/433477/Leukocyte-Disorders
Leukemia, leucopenia and infectious mononucleosis are examples of leukocyte disorders. The overproduction or underproduction of leukocytes all lead to serious complications.
LeukemiaLeukemia - Leukocyte DisordersCredit: http://healorhell.com/disease-leukemia-blood-cancer/
Leukemia, which literally translates to white blood, is referring to a group of cancerous condition, all involving the white blood cells. The abnormal leukocyte is from a single clone which failed to specialize and is rapidly proliferating, thereby impairing the bone marrow to create healthy and mature leukocytes. Acute leukemia happens if the abnormal leukocyte involved came from blast type cells like lymphoblasts. Chronic leukemia on the other hand involves proliferation of the later cell stages like myelocytes. As what have seen by previous studies, acute leukemia primarily affects children and chronic leukemia in elderly people. The sad part in the story of leukemia is that all are fatal.
The abnormal proliferation of unspecialized leukocytes almost fill up the bone marrow, thereby impeding its ability to create healthy RBCs and WBCs. As a result, immature white blood cells are pushed out into the bloodstream. Although a number of leukocytes are produces, they still are not able to function; hence they don’t have the ability to defend the body.
Infectious Mononucleosis
 Infectious Mononucleosis - Leukocyte DisordersCredit: http://hardinmd.lib.uiowa.edu/cdc/mononucleosis1.html
Interestingly, this disease was once referred to as the kissing disease. Caused by the Epstein-Barr virus, this is a very contagious viral disease. Up to date, no cure has been found for this disease but with adequate rest, recovery will be evident in a few weeks.