" The body uses a code system called the HLA (human leukocyte antigen) system, which works in a similar fashion. It gives a code to all surfaces. Immune cells basically identify surfaces. Everything has a surface, whether it is your own cells, a microorganism, or a piece of food. When your immune system scans the interior surfaces of your body, it compares each to a list of approved codes, the ones it classifies as “self. "

― Alejandro Junger , Clean Gut: The Breakthrough Plan for Eliminating the Root Cause of Disease and Revolutionizing Your Health