Final answer:
The disorder characterized by a white blood cell count below four thousand cells per microliter is leukopenia, a condition leading to a reduced ability to ward off diseases.
Step-by-step explanation:
The disorder called when the white blood cell count is below four thousand cells per microliter is leukopenia. Leukopenia involves a decrease in the production of leukocytes (white blood cells), which are crucial for the body's immune system to fight off diseases. Understanding the different types of white blood cells and their counts can be important for diagnosing various conditions, including leukopenia, which can leave individuals at an increased risk for disease due to a compromised immune system.