Leukopenia indicates a decrease in the body's resistance. The most common causes of leukopenia are: First, viral infections, which often coincide with an increase in lymphocytes and a decrease in neutrophils. The second cause is severe bacterial infections. During severe bacterial infections, there is a significant reduction in white blood cells, which may even lead to a decrease in platelets. The third cause is primarily diseases of the hematologic system, where leukopenia is often accompanied by a reduction in other red blood cells and platelets. Other possible causes include drug-induced leukopenia, as medications can also lead to a decrease in white blood cells. Therefore, the primary approach to leukopenia is to clearly identify the cause, and then determine the next steps for treatment based on that cause.