White blood cell, also called leukocyte or white corpuscle, a cellular component of the blood that lacks hemoglobin, has a nucleus, is capable of motility, and defends the body against infection and disease by ingesting foreign materials and cellular debris, by destroying infectious agents and cancer cells, or by producing antibodies.


A healthy adult human has between 4,500 and 11,000 white blood cells per cubic millimetre of blood. Fluctuations in white cell number occur during the day; lower values are obtained during rest and higher values during exercise. 


An abnormal increase in white cell number is known as leukocytosis, whereas an abnormal decrease in number is known as leukopenia. 


White cell count may increase in response to intense physical exertion, convulsions, acute emotional reactions, pain, pregnancy, labour, and certain disease states, such as infections and intoxications. The count may decrease in response to certain types of infections or drugs or in association with certain conditions, such as chronic anaemia, malnutrition, or anaphylaxis.


Although white cells are found in the circulation, most occur outside the circulation, within tissues, where they fight infections; the few in the bloodstream are in transit from one site to another. 


As living cells, their survival depends on their continuous production of energy. The chemical pathways utilized are more complex than those of red blood cells and are similar to those of other tissue cells. 


White cells, containing a nucleus and able to produce ribonucleic acid (RNA), can synthesize protein. White cells are highly differentiated for their specialized functions, and they do not undergo cell division (mitosis) in the bloodstream; however, some retain the capability of mitosis. 


On the basis of their appearance under a light microscope, white cells are grouped into three major classes:


  • Lymphocytes
  • Granulocytes, and 
  • Monocytes


Each of which carries out somewhat different functions.


Granulocytes, the most numerous of the white cells, rid the body of large pathogenic organisms.


Granulocytes are subdivided into three categories: neutrophils, eosinophils, and basophils. The most numerous of the granulocytes (making up 50 to 80 percent of all white cells) are neutrophils. They are often one of the first cell types to arrive at a site of infection, where they engulf and destroy the infectious microorganisms through a process called phagocytosis: