Leukocytes (also leucocytes, white cells, white blood cells, or white blood corpuscles) are cells of the immune system that rid the body of debris and protect it against contagious disease. They arise from the same cells (hematopoietic stem cells), residing in the bone marrow, that can alternatively mature into erythrocytes. They are able to enter tissue and return to the blood stream.
Normally there are between 4×10⁹ and 1×10¹⁰ white cells in a liter of human blood. Leukocytosis, or leukopenia, is the name given to the condition where white cells are present at an elevated level, which is a frequent occurrence in disease.
Pus is composed of dead white cells that have been overcome by bacteria. The term "white blood corpuscle" is derived from the fact that these cells show up in centrifugated blood as a white layer.
There are several types of white cells:
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