Leukocytes are white blood cells, the part of blood system that are involved in immune system defense. When the blood test is high, it implies stimulation, usually with infection. Low leukocytes implies immune system supression, often from chemotherapy, but from other causes such as bone marrow dysfunction, toxicities, and other unknown causes.

Leukopenia-Wikipedia

Specific types of white cells can be individually stimulated or suppressed.