White blood cell are immune system cells that defend the body against infectious diseases and foreign substances. They are also known as leukocytes. There are five specific types of white blood cells (along with their approximate percentage):

neutrophil (50-70%)
eosinophil (3-4%)
basophil (0.5-1%)
monocyte (3-8%)
lymphocyte (25-45%)

Leukocytes are named white blood cells due to the "buffy coat" found in samples of anticoagulated blood. In such a sample, the clear plasma (roughly 55%) rises to the top, while the red blood cells, or erythrocytes, (roughly 45%) sink to the bottom. The leukocytes and platelets (less than 1%) remain in middle, forming a white layer.

The amount of white blood cells in the blood can often be used as a diagnostic tool for certain diseases. For instance, in leukemia, the white blood cell count is above the normal range, while in leukopenia, it is lower. Particular physical characteristics of the white blood cells may also be altered in leukemia.

Instead of staying in the blood, certain white blood cells permanently move into body tissue. These are known as fixed leukocytes, and are often known by other names based upon the location that they end up at. Some specific types of fixed leukocytes are: dendritic cells, histocytes, Kupffer cells, mast cells, and microglia.

Certain medications may cause leukopenia (low white blood cell count) and/or altered function of the white blood cells.