The symptoms of leukopenia vary with the severity of the decrease in white blood cells and the disease causing the leukopenia. Mild leukopenia generally has no significant clinical manifestations. With moderate leukopenia, patients often experience symptoms of fatigue. If there is a significant decrease in white blood cells, patients often present with fever since white blood cells are the main cells in the body's defense against infections. A significant decrease in white blood cells markedly lowers the patient's resistance, thus making them more susceptible to various infections, with fever being the most common symptom of infection. Furthermore, the clinical presentations also vary with the underlying disease causing the decrease in white blood cells. For example, diseases like aplastic anemia or acute leukemia, which cause a decrease in white blood cells, may lead not only to fever but also to significant fatigue and bleeding manifestations.