Autoimmune diseases
Autoimmune diseases have a common mechanism: the immune system goes mad and starts attacking the body cells. What is strange about these diseases is that sometimes the immune system targets cells of the immune system, including white blood cells, causing a fall in their numbers. This may be significant in some diseases, such as systemic lupus, where a decreased white blood cell count is considered an important criterion for the diagnosis.
Other autoimmune diseases associated to leukopenia are rheumatoid arthritis and Sjogren syndrome. What is even worse is that the treatment of such conditions starts with immune suppression, causing further deterioration of white blood cells in many cases.