The purpose of this review is to demonstrate that an intestine leaky to small molecules can be impermeable to large antigenic molecules. The author proposes that the permeability of the epithelium to very small sugar molecules such as lactulose/mannitol-used for the past 50 years to gauge intestinal permeability-does not necessarily correlate with epithelial permeability to macromolecules. This article begins with the history and science behind the use of small sugars to measure permeability, a method developed in 1899. The lactulose/mannitol test may give useful information regarding the overall condition of the digestive tract; however, the author suggests that the test is not indicative of the transport of macromolecules such as bacterial toxins and food antigens, which have the capacity to damage the structure of the intestinal barrier and/or challenge the immune system. This article describes the various mechanisms and physiological transport pathways through which increased antigen uptake may result in immunological reactions to food antigens and bacterial lipopolysaccharides, resulting in the pathogenesis of disease. Finally, the article presents evidence indicating that increased intestinal, antigenic permeability plays a key role in the development of various inflammatory and autoimmune disorders. Therefore, more knowledge about the epithelium’s permeability to large molecules undoubtedly contributes not only to early detection but also to secondary prevention of many inflammatory autoimmune, neuroimmune, and neurodegenerative disorders. (Altern Ther Health Med. 2013;19(1):12-24.).