The immune system is a collection of organs, cells and specialized tissue that work together to defend your body against harmful bacteria and viruses (pathogens). One of the most fascinating abilities of a properly functioning immune system is that, while defending against dangerous pathogens, it can determine between what is supposed to be in the body and what is foreign and should be attacked. The picture above illustrates some of the major parts, but actually some are not shown. These other important components are your tonsils, appendix, heart and stomach which all function to make this defense system work exceptionally well. As we begin our tour of the system, let's start in a place you might not expect: the blood.