Intestine: organ or apparatus?

It is well known that human intestine is involved in different important functions. First of all, it is responsible for digestion and absorption of nutrients, electrolytes, water, bile salts and drugs, but it also has immunologic, endocrine and motor functions. Moreover, intestinal microflora, composed by a large diversity of bacterial cells, provides several beneficial functions for the host and is, nowadays, defined by many authors as an organ itself. In consideration of intestine complexity, we tried to understand if it can be considered only an organ or if it is an apparatus itself. We have analyzed the different components and their relationships, showing that a continuous collaboration is required among enterocytes, endocrine intestinal cells, gut immune system and microflora to assure an efficient mechanism of defense. In consideration of the complexity of intestinal components, together with the emergent role of microflora, we think that we could start to consider gut as a real apparatus, and not only as an organ.