When infections or acute stress cause the production of white blood cells to increase, an increase in the percentage of immature cells also occurs, and this change is what is referred to as a "shift to the left," according to MedlinePlus. Band zcells make up most of the immature cells.
There are many different types of white blood cells, and physical or emotional stress can cause the amount of white blood cells in the body to increase. Different types of drugs can also increase white blood cell counts, including aspirin, heparin, corticosteroids, epinephrine, chloroform and quinine. Drugs that can decrease white blood cell counts include antihistamines, antibiotics, barbiturates and chemotherapeutic agents. MedlinePlus notes that high numbers of white blood cells indicate that an infectious or inflammatory disease might be present, and low numbers may indicate bone marrow failure or a disease of the liver or spleen.