Have you ever taken a ride on a waterslide at an amusement park? If so, you've got a pretty good idea of how your blood travels through your body. While it is traveling around, it has some important jobs to do. Check this out:

Blood contains four major parts:

• Red blood cells - carry oxygen (O2) from your lungs to your body cells and carbon dioxide (CO2) from your body cells back to your lungs to be exhaled
• Platelets - help clot blood
• White blood cells - fight germs that infect the body
• Plasma - a yellowish liquid that consists mostly of water

• A, B, AB and O blood types are determined by the presence or absence of antigens (specific chemicals) on the red blood cells
• Negative (-) or positive (+) blood types are determined by whether or not a chemical called the Rh factor is present on the red blood cells. If someone has this chemical on his red blood cells, then his blood type is positive (+), if he doesn't, then his blood type is negative (-)

Immune System:
Your immune system is like an army that proctects you from disease.

• Your skin is the first wall of protection against foreign invaders that cause disease.
• The second line of defense are fluids like mucus found in your respiratory system and tears from your eyes.

If the invaders do pass through these defenses there is an army battalion of white blood cells and their weapons that fight the germs:

• Helper Tcells - act as the "lookout" for your body by recognizing invaders and then send signals to the other white blood cells
• Bcells - make antibodies to smother the invaders once they receive the signals from the helper Tcells
• Killer Tcells - kill the invaders once they receive the signals from the helper Tcells
• Phagocytes - act as "eating" cells. They destroy invaders with chemicals and then eat them.