White blood cells protect the body against infection. Types of white blood cells include neutrophils, banded neutrophils, lymphocytes, monocytes, and eosinophils.

Each type of white blood cell plays a different role in protecting the body. The numbers of each type of white blood cells give important information about the immune system. Too many or too few of the different types of white blood cells can be a sign of an infection, an allergic or toxic reaction to medicines or chemicals, and many conditions, such as leukemia.