White blood cell types (WBC differential). There are five major kinds of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Immature neutrophils, called band neutrophils, are also included and counted as part of this test. Each type of cell plays a different role in protecting the body. The numbers of each one of these types of white blood cells give important information about the immune system. An increase or decrease in the numbers of the different types of white blood cells can help identify infection, an allergic or toxic reaction to certain medications or chemicals, and many conditions (such as leukemia).
This is part of the CBC – Complete Blood Count Test.
Other definitions of WBC on the web found here.