Leukocytes or white blood cells (WBC) are cells in the blood that are involved in defending the body against infective organisms and foreign substances. Like all blood cells, they are produced in the bone marrow. There are 5 main types of white blood cell, subdivided between 2 main groups:

1. Polymorphonuclear Leukocytes (granulocytes):

  Neutrophils
  Eosinophils
  Basophils

2. Mononuclear Leukocytes:

  Monocytes
  Lymphocytes

White blood cells are the principal components of the immune system and function by destroying "foreign" substances such as bacteria and viruses. When an infection is present, the production of WBCs increases. If the number of leukocytes is abnormally low (a condition known as leukopenia), infection is more likely to occur and it is more difficult for the body to get rid of the infection.

White Blood Cell Count . Measures the number of white blood cells in a microliter of blood. Normal values range from 4100/ml to 10900/ml but can be altered greatly by factors such as exercise, stress and disease. A low WBC may indicate viral infection or toxic reactions. A high WBC count may indicate infection, leukemia, or tissue damage. An increased risk of infection occurs once the WBC drops below 100/ml.

WBC Differential . Determines the percentage of each type of white blood cell in a sample. Multiplying the percentage by the total count of white blood cells indicates the actual number of each type of white blood cell in the sample.
Normal values are:

Type	Percentage	Number
Neutrophil Eosinophils Basophils Lymphocytes Monocytes	50 - 60% 1 - 4% 0.5 - 2% 20 - 40% 2 - 9%	3,000 - 7,000 50 - 400 25 - 100 1,000 - 4,000 100 - 600

A serious infection can develop once the total neutrophil count (% neutrophils times total WBC count) drops below 50/ml.