Complex sentence: The microbiome of the gut has been characterised as a "forgotten organ", and the possibility has been raised that "the mammalian immune system, which seems to be designed to control microorganisms, is in fact controlled by microorganisms".
Split into simpler sentences: The microbiome of the gut has been characterised as a forgotten organ ", O'Hara, A. M. and Shanahan, F. (2006)" The gut flora as a forgotten organ. PMID 16819463 and the possibility has been raised that "the mammalian immune system, which seems to be designed to control microorganisms, is in fact controlled by microorganisms".