White blood cells are blood components that protect the body from infectious agents. Also called leukocytes, white blood cells play an important role in the immune system by identifying, destroying, and removing pathogens, damaged cells, cancerous cells, and foreign matter from the body. Leukocytes originate from bone marrow stem cells and circulate in blood and lymph fluid. Leukocytes are able to leave blood vessels to migrate to body tissues. White blood cells are categorized by the apparent presence or absence of granules (sacs containing digestive enzymes or other chemical substances) in their cytoplasm. A white blood cell is considered to be a granulocyte or an agranulocyte.