White blood cells are made in the bone marrow and protect the body against infection. If an infection develops, white blood cells attack and destroy the bacteria, virus, or other organism causing it.

White blood cells are bigger than red blood cells and normally are fewer in number. When a person has a bacterial infection, the number of white cells can increase dramatically.

The white blood cell count shows the number of white blood cells in a sample of blood. A normal white blood cell count is between 4,500 and 11,000 cells per cubic millimetre (4.5 and 11.0 x 10⁹ cells per litre). The number of white blood cells is sometimes used to identify an infection or to monitor the body's response to treatment.

There are five types of white blood cells: lymphocytes, monocytes, neutrophils, basophils, and eosinophils.