Lec.4:Leukocytes

Leukocytes (White Blood Cells)
Leukocytes are the only formed elements that are complete cells, with nuclei and the usual organelles. Accounting for less than 1% of total blood volume, leukocytes are far less numerous than red blood cells. On average, there are 4800–10,800 WBCs/?l of blood.
Leukocytes are crucial to our defense against disease. They form a mobile army that helps protect the body from damage by bacteria, viruses, parasites, toxins, and tumor cells. As such, they have special functional characteristics. Red blood cells are kept into the bloodstream, and they carry out their functions in the blood. But white blood cells are able to slip out of the capillary blood vessels in a process called diapedesis and the circulatory system is simply their means of transport to areas of the body (mostly loose connective tissues or lymphoid tissues) where they mount inflammatory or immune responses.

The signals that prompt WBCs to leave the bloodstream at specific locations are cell adhesion molecules displayed by endothelial cells forming the capillary walls at sites of inflammation. Once out of the bloodstream, leukocytes move through the tissue spaces by amoeboid motion (they form flowing cytoplasmic extensions that move them along). By following the chemical trail of molecules released by damaged cells or other leukocytes, a phenomenon called positive chemotaxis, they pinpoint areas of tissue damage and infection and gather there in large numbers to destroy foreign substances and dead cells.
Whenever white blood cells are mobilized for action, the body speeds up their production and their numbers may double within a few hours. A white blood cell count of over 11,000
cells/?l is leukocytosis. This condition is a normal homeostatic response to an infection in the body.