The average human body contains about 60 trillion cells, and each of them has work to do! But when you get injured, viruses or bacteria invade, or when an allergic reaction flares up, everyone from the silent but deadly white blood cells to the brainy neurons has to work together to get through the crisis! Strep throat! Hay fever! Influenza! The world is a dangerous place for a red blood cell just trying to get her deliveries finished. Fortunately, she's not alone... she's got a whole human body's worth of cells ready to help out!