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White Blood Cells, Platelets (stained purple), a T-Lymphocyte white cell (stained green), and a Monocyte white cell (stained gold) as seen through a scanning electron microscope. ©2000 Dennis Kunkel, Ph.D.

The scientific name for white blood cells is leukocytes. Because white blood cells are hard to see when scientists look for them under a microscope, they are usually stained with a bright colored dye. Just like red blood cells, white cells are formed in the bone marrow and are created by a parent cell called a stem cell.

White blood cells are an important part of the body's immune system. Their role is to defend the body against infection by germs.  There are many different kinds of white blood cells and each one has a very specific job to do. There are lymphocyte T cells and lymphocyte B cells, monocytes, and granulocytes.


White blood cells are capable of passing through the walls of capillaries (tiny blood vessels) in order to attack, kill and consume intruder germs. Granulocytes contain little granules in their cytoplasm or cell matter.  Granulocytes can be identified even further as neutrophils, basophils and eosinophils. Granulocytes recognize signals that germs send out when they invade the body.

Monocytes and lymphocytes do not contain any granules.  But when granulocytes detect an invading germ, the lmphocytes and monocytes find it and eat it. Then the monocyte examines the bits of protein the germ was made of to see how it was put together.  Next, the monocyte calls on the lymphocyte T cell (or Helper T cell) which learns to recognize what the germ looks like.  The lymphocyte T cell then engages the help of the lymphocyte B cell which makes a special weapon called an antibody to use against the germ. The lymphocyte B cell produces copy after copy of these antibody weapons. When the antibody weapon finds its target, the germ is stunned, wounded and killed. Then the granulocyte and monocyte move in to finish it off.  There are between 7,000 to 25,000 white cells in a single drop of blood.

The coolest blood cell is the white blood cell which fight the germs.
— Katherine,
high school student
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