Leukocyte disorders can lead to either leukopenia, characterized by an abnormally low leukocyte count, or leukocytosis, marked by a very high leukocyte number.

Leukopenia may result from bone marrow disorders, autoimmune diseases, and infectious diseases. For example, conditions such as multiple myeloma and aplastic anemia can impair the bone marrow's ability to produce adequate leukocytes. Similarly, autoimmune diseases like lupus and viral infections such as HIV can prompt the immune system to attack cells involved in leukocyte production.

Conversely, leukocytosis typically occurs as a natural response to infections. When the body detects an invading infectious agent, it increases the release of leukocytes into the bloodstream to combat the threat. However, an extremely high leukocyte count, exceeding 100,000 cells per microliter, usually indicates a form of leukemia. These blood cancers are categorized based on the speed of their progression and the affected cell lineage.

For instance, acute myeloid leukemia is a rapidly spreading cancer of the myeloid lineage, whereas chronic lymphocytic leukemia progresses slowly and affects lymphoid cells.

Infectious mononucleosis, often caused by the Epstein-Barr virus, can increase atypical lymphocytes, which may be mistaken for monocytes.