Leukocytes, also known as white blood cells, are the cells of the immune system. The role of leukocytes is to fight infection by destroying the bacteria, viruses and other foreign organisms (pathogens) which are responsible for it. They do this in a number of ways that can be broadly divided into the acquired immune system which recognises specific invaders and the innate immune system which does not recognise specific invaders. In fact the two systems closely interact with eachother and the division is far from total.
Leukocytes communicate with eachother and with other cells of the body via chemical messagers called cytokines.
All leukocytes originate in the bone marrow, though some are matured in other sites such as the thymus (T-cells) and spleen.
Leukocytes divide up into five types within two classes: