White blood cells play an important role in your body’s immune system, searching the blood for invading viruses, bacteria, and fungi. When a foreign virus or bacteria enters your blood, the white blood cell, or leukocyte, recognizes and destroys the invading particle before it can cause disease. There are several different types of white blood cells, each with their own function. Some directly destroy the foreign bacteria, while others attack cells that are infected by viruses. Other types of ...