White blood cells, also known as leucocytes, are a vital part of the body’s immune system; they defend the body against infectious agents (pathogens) and other foreign substances. They are much fewer in number than the red blood cells: in a healthy state between 5.0 and 11.0 million leucocytes per millilitre of blood. There are 5 different types of leucocytes in the blood. Three types can be grouped together as granulocytes which are all similar in appearance (neutrophils, basophils and eosinophils). They are also known as small scavenger cells. The other two types are lymphocytes and monocytes, also known as larger scavenger cells.

When the need arises, e.g. when the body detects invasion of an infectious agent, white blood cells can actively leave the circulatory system by moving out of the blood vessels into the tissue. Once in the tissue, the leucocytes can locate and destroy invading pathogenic organisms and so prevent an infection from spreading. Each type of leucocyte has its own specific task; the neutrophil granulocytes for instance play an important role in the fighting of bacterial infections.