Owing to the fact that, until recently, all of the infectious pathogens suggested as the trigger of TAO were gram-negative bacteria, the gene expression of toll-like receptor 4 (TLR4) as the receptor of innate immunity for the main PAMPs of gram-negative bacteria, so-called lipopolysaccharides (LPS), was evaluated in the sympathetic ganglia of TAO patients. This evidence concerns the gene TLR4 and thromboangiitis obliterans.