Bodily Defenses Image Map
Immune System
Remember the example of the splinter from the introduction?  The body considers a splinter (and the bacteria on it) as a foreign invader. When such invaders are detected, specific defense cells are mobilized and travel through the blood and lymph vessels to seek out the invader and fight it. These cells are called white blood cells, because they are, in fact, white cells in the blood. One type of white cell is the lymphocyte, which is made in many lymph nodes scattered throughout the body.
The photo on the left shows a magnified view of a slice of lymph node. The circular zones contain lymphocytes that have been created by the lymph node. A good diagram of a lymph node and the circulation of lymph fluid can be seen by clicking here.
The photo on the right shows an even higher magnification of a lymph node. The black dots are tiny white blood cells, lymphocytes, and the clear areas are the blood and lymph vessels that will carry the lymphocytes to other parts of the body.

Lymph nodes are connected together by way of their own system of vessels, which eventually drain into veins near the heart. Altogether, we refer to that as the lymphatic system. To see a diagram of this system, click here.

The first challenge for the immune system is to know that the skin's first line of defense has been penetrated. So how does the immune system know that an invasion has occurred and that invaders have penetrated the skin? The answer is that certain cells of the immune system recognize that the proteins of the bacteria are foreign. 

Cells of your body have proteins on their surface that the immune system recognizes and leaves alone.   But proteins that are not recognized stimulate the immune system to make antibodies, which are proteins that attach to the foreign proteins and make them inactive. Many antibodies circulate in the blood, while others are anchored in cells. The free-circulating antibodies can be transferred from one individual to another by blood transfusions or from mother to newborn via the milk for the first day or two after birth. The antibodies attached to cells can only be transferred by transferring the entire cell, which is makes them less transferable since the recipient's own immune system would attack the "foreign" cells.

Now, let us learn more about antibodies.


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