The generic function of leukocytes is to participate in the defense of the body against strange agents that penetrate it or are made inside the body. Leukocytosis and leukopenia are clinical conditions in which the count of leukocytes in a blood sample is abnormal. When the leukocyte count in a blood sample is above the normal level for the individual leukocytosis is defined. When the leukocyte count is lower than the expected normal level leukopenia is defined. The multiplication of these defense cells, leukocytosis, generally takes place when the body is suffering infections or in cancers of these cells. The lowering of these defense cells, or leukopenia, occurs when some diseases attack the cells, like in AIDS, or when immunosuppressor drugs are used. In general the body creates leukocytosis as a defense reaction when it is facing infectious or pathogenic agents. The clinical condition of leukocytosis is thus a sign of infection. Leukopenia occurs when there is a deficiency in the production (for example, in bone marrow diseases) or excessive destruction of leukocytes (for example, in case of HIV infection).