definition of leukocytosis

[Michel Auffret]

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A question from a scientist involved in invertebrate immunology:

Could somebody give me a definition for the so-called phenomena: leukocytosis and leukopenia?
Especially, is it correct to use these terms for respectively an increase or a decrease in total circulating leukocyte numbers?
Do these phenomena necessary imply some kind of involvement of hemopoietic organs?
Thank you