White Blood Cells

The other name for white blood cells is leukocytes. These cells have a very different purpose than RBCs... they have to deal with infection, and they also have to clean up the body (removing debris, infectious agents, and toxins). Therefore, these cells simply traverse the circulatory system to encounter the things they need to deal with. They do not need to travel and travel and travel around the body like the RBCs.

How do they get out of the blood vessels to enter troubled tissue? They use a method that is called diapedesis, but it simply means that they creep out of the blood vessels called capillaries by crawling between capillary wall cells. You see, capillaries are very thin-walled blood vessels... in fact, the walls are only one squamous cell in thickness (simple squamous epithelium). Therefore, capillaries could have holes in them at the site where the squamous epithelial cells meet, as long as these cells don't meet very tightly. In the capillaries of most regions of the body, there really are tiny spaces available between these epithelial cells. Can you think of somewhere that there must be no holes between the capillary wall cells? Think back to a something-or-other barrier you learned about last semester...

That's right. The blood-brain barrier. So capillaries are not traversible in the central nervous system. But other than that, the epithelium of the capillary has tiny spaces available. That doesn't mean that fluid leaks out or anything. It's more like the cells overlap, but a cell could push its way through where they overlap.

Anyway, white blood cells can creep out of the blood by diapedesis. When they do, their creeping is simply a form of amoeboid locomotion... it's the same way amoebas, macrophages, fibroblasts, cancer cells, many embryonic cells, and other cell types move.

You know what the different types of white blood cells are, at least generally. Now it's time to go over that and also to fill you in on what they do.

Description by appearance:

	granulocytes: these all contain granules in their cytoplasm, visible with simple staining. They don't live long-- only about 12 hours! What are the granules? Well, they are either secretory vesicles (implying that some of the cells secrete materials) and others are just very busy lysosomes (even though agranulocytes can also have busy lysosomes, they don't stain the same way).
	agranulocytes: cells without granules in their cytoplasm.

Description by function:

You will see that these cells generally fit into one of three modes of function-- phagocytosis, secretion, immunity. This is a big overgeneralization, but works for a start. By phagocytosis, I mean that the cell ingests materials to destroy them or remove them from where they had been. Let's say a cell in the connective tissue dermis dies. Another cell originating as a white blood cell would then come in and phagocytize the cellular debris, removing it from the dermis. Another type of phagocytosis would be to phagocytize bacteria. By secretion, I mean that some white blood cells tend to secrete chemicals to cause local (not blood-borne) signals. A white blood cell could secrete a chemical to cause inflammation in a particular tissue, or to stop inflammation. Inflammation is not a bad thing, or we wouldn't do it! Inflammations help us get blood (and white blood cells) to damaged tissues. As far as immunity goes, we're going to leave that for the immune system unit, but it just is a means to help us fight infection.

Please note that white blood cells can do a little bit of each of those functions above. Here's a more specific list:

Anatomical Type	Specific Cell Type	Function
granulocyte	neutrophil	Major Function: Phagocytosis Neutrophils are very good at phagocytosing smaller chunks of material, like bacteria. They can also secrete chemicals to enhance an inflammatory response. They are also targeted to the bacteria they destroy by the immune system.
	eosinophil	Major Functions: Secretion of toxic materials to kill parasites and other invaders, as well as phagocytosis of bacteria. These cells are particularly good at fighting off parasitic invasions. They, like neutrophils, are also targeted to bacteria by the immune system. They also secrete chemicals in allergic reactions.
	basophil	Major Function: Secretion These cells secrete both histamine and heparin. Both of these chemicals promote the inflammatory response, but in different ways. Histamine draws blood into the damaged area, while heparin slows clotting so that more and more blood can still enter the damaged area.
agranulocyte	monocyte	Major Function: Phagocytosis Monocytes are only found in the blood. As soon as they use diapedesis to enter tissues, they are called macrophages. These cells (that you learned about in the connective tissue chapter) crawl around and phagocytize all sorts of things-- big or small. They are the ones that pick up cellular and tissue debris.
agranulocyte	lymphocyte	Major Function: Immunity There are different types of lymphocytes. Some secrete toxic chemicals, others are more directly involved in an immune response. We will get to these in the immune system unit.

For a more complete understanding of how some of these cells work, you will need to understand how our blood deals with injury. That is described on the "damage control" page. It will help if you first learn about platelets.

© 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.