What are white blood cells?

White blood cells are responsible for protecting the body from invasion by foreign substances, such as bacteria and viruses. The majority of white blood cells are produced in the bone marrow, where they outnumber red blood cells by 2 to 1. However, in the blood stream, there are about 600 red blood cells for every white blood cell.

 There are several types of white blood cells. Granulocytes and monocytes protect against infection by surrounding and destroying invading bacteria and viruses, while lymphocytes aid in the immune defense system.

White blood cells are probably the most important part of the immune system. "White blood cells" are actually a vast collection of different cells working together to destroy bacteria and viruses. The body has more than a dozen types of white blood cells.

All white blood cells are known as leukocytes. White blood cells are not like other cells in the body - they act like independent, living single-cell organisms. They are able to move on their own and can engulf other cells and bacteria. Many white blood cells can't divide and reproduce on their own. Instead, they are produced in the bone marrow.

Leukocytes are divided into three classes:

• Granulocytes, which make up 50 to 60 percent of all leukocytes. Granulocytes get their name because they contain granules, which contain different chemicals depending on the cell.
• Lymphocyte, which make up 30 to 40 percent of all leukocytes. Lymphocytes can be divided into two classes: B cells (that mature in bone marrow) and T cells (that mature in the thymus).
• Monocyte, which make up more or less 7 percent of all leukocytes. Monocytes evolve into macrophages.

Every white blood cell starts in bone marrow as a stem cell. Stem cells are generic cells that - as they mature - become the many different leukocytes. For example, a scientist can take a mouse and irradiate it to kill off the bone marrow's ability to produce new blood cells. The scientist can then inject stem cells into that mouse's blood stream. The stem cells will divide and become all the different types of white blood cells.