2. As it circulates throughout the body, red blood cells carry oxygen to all parts of your body and they remove carbon dioxide. However, did you know that the blood has more than just one type of cell? The most abundant are red blood cells that carry oxygen. There are also little cell-like pieces called platelets that are essential in blood clotting. One of the most fascinating components of blood, are the white blood cells. These can be further divided into five types of cells which defend the body against bacteria, viruses and parasites. The blood cells are suspended in plasma, which consists mostly of water with clotting factors, cell nutrients, sugar, and hormones.

3. Marrow is a yellow-white tissue in the center of many bones. It's the location of a type of stem cell (specifically, the pluripotential hemopoietic stem cell), from which come the many types of blood cells. It's a marvelous biological feat that all the cells in our blood -- red, white and the platelets -- come from one type of cell that develops into such a variety of results.

4. Here are drawings of the cells in your blood. The familiar red blood cell, pictured in the lower right, is the erythrocyte. It was once a living cell, but by the time it gets to your bloodstream it's only a "biological box" that can carry large amounts of oxygen. On the lower left are the platelets. When you have a cut, the platelets bind together and form a blood clot that stops the bleeding.

5. The white blood cells are a collection of five types of cells that patrol the blood stream and tissues of the body looking for bacteria, viruses and parasites. Together they are known as white blood cells or leukocytes. The most common type of leukocyte is the neutrophil and is the body's front line of defense against harmful bacteria. You have probably seen evidence of neutrophils if you've ever had a cut that got infected. Surrounding the infection is usually a fluid called "pus" which mostly contains the remains of dead neutrophils.

6. Monocytes will patrol the bloodstream for a little while, but soon develop into macrophages that can actually "eat" bacteria in the body that isn't supposed to be there. Because of this, macrophages are large and able to engulf the invader.

7. Basophils are the least common white blood cell. In addition to fighting bacteria, they are involved in releasing histamine, a biochemical that ultimately leads to an increase in swelling. Not pictured are lymphocytes, which are the second most common type of leukocytes. Lymphocytes are commonly found in the blood, but also in the lymphatic system. They develop into either B-lymphocytes or T- lymphocytes and have many functions in fighting bacteria and viral infections.

8. Eosinophils are especially valuable as fighters of parasites. Because of this association, doctors may suspect a parasitic infection if you have a blood test that shows a higher number of eosinophils in the blood than normal.

9. Remember the white blood cell called the monocyte? At some point in its development, it becomes a macrophage, which is Greek for "Large Eater." It's kind of like a powerful garbage disposal that is useful to our immune system because it can actually eat (the process of phagocytosis) foreign invaders, whether microbial or not. Once the phagocyte surrounds the bacterium, it will destroy it by "digesting" it. Macrophages patrol the body looking for anything it can "eat." However, macrophages are particularly on the lookout for anything marked with a "special sign." Other white blood cells, especially the lymphocytes, attach antibodies to foreign microbes which effectively identifies the germ as something bad that needs to be destroyed.

10. Maybe less familiar is the distribution of another network of vessels that are similar to veins, but not as extensive. Instead of transporting blood, they carry a clear fluid called lymph. As nutrients seep from the blood into the tissues, the lymphatic system collects this fluid along with any associated wastes and returns it to the blood. Lymph is a great place to fight microbes and it's filled with lymphocytes and other white blood cells. Before the lymph gets recycled into the bloodstream, lymphocytes work to identify any harmful microbes so they can be destroyed.

11. Along the lymphatic system are collections of specialized tissue called lymph nodes. These are places where large amounts of lymphocytes stay, which can attack any microbes found in the lymph as it filters through the lymph node.

12. The spleen is an oval-shaped organ located in the upper-left side of your belly, between the stomach and diaphragm. It's where old, worn-out blood cells go to be recycled. However, since it's also where bacteria are filtered out of the blood, it's the largest single organ of the immune system. Oddly enough, the spleen is a non-essential organ. You can actually live without it, but you'll be more likely to get certain types of bacterial infections.

13. The thymus, located between your lungs and behind your sternum, is where T-lymphocytes develop. Though these white blood cells begin from stem cells in bone marrow, they further specialize into T-lymphocytes here. The "T" actually stands for "thymus" to reflect this origin. The thymus is an interesting organ: Although it's active in young people and teenagers, it shrinks and becomes much less active in adults.