ABSTRACT
The protective functions of immune defense are built upon mechanisms for distinguishing the constituents of infectious agents from the constituents of the human body. The immune system is thus able to be tolerant of self but recognize that pathogens and their antigens are foreign. Apart from infectious agents, humans come into daily contact with numerous macromolecules that are equally foreign but do not in any way threaten their health. Many of these potential antigens derive from the plants and animals that provide our food or are present in the environments where we live, work, and play. For most people for most of the time, contact with these molecules stimulates neither inflammation nor adaptive immunity. In these circumstances the immune system is seen to distinguish what is dangerous from what is not.
