Bodily Defenses Image Map
We have already seen how white blood cells will attack, swallow, and digest an invader.  But what are these white blood cells and how do they work?

White Blood Cells: neutrophils & lymphocytes
There are five types of white blood cells, but only two are of importance in this discussion. One type is the neutrophil, which accounts for about 60% of all white cells. During infections, these cells divide and are dumped into the blood stream and lymph fluids to combat the infection. They fight invaders by "eating" bacteria and other foreign matter. The neutrophil membrane wraps around the invader and traps it inside the neutrophil. Once inside, digestive enzymes in the neutrophil destroy the invader.
The cell nucleus in new, young neutrophils tends to break into pieces as they age. Thus, by counting the percentage of neutrophils that have a single, intact nucleus, a doctor can know that the body is responding as expected to an infection.

On the right, two white blood cells surrounded by red blood cells. The one indicated by the arrow is a mature neutrophil. The one just to the left is a lymphocyte.
On the left, three mature neutrophils at the bottom, a lymphocyte near the top, and a monocyte (arrow).  The monocyte matures into a large macrophage cell.

The other important type of white cell is the lymphocyte, which comes in two forms, a B-type and an T-type. The B-type makes circulating antibodies. The B-type is made in lymph nodes and bone marrow.

The T-type lymphocyte ("T" is for killer) either kills foreign cells on contact or helps by releasing chemicals that assist in killing invaders.
T-cells recognize the surface proteins on other cells and when they come in contact with an intruder, the invading cells are killed. These lymphocytes are made in a gland at the base of the neck, called the thymus.  This gland is very large in young people, but almost disappears in old age.  Production of T-cells can be reactivated at any time, if a foreign antigen is presented. T-cells also play a role in the rejection of transplanted tissue and in fighting cancer.


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