“The triggering receptor expressed on myeloid cells (TREM-1) is a member of the immunoglobulin superfamily17 whose expression on phagocytes is up-regulated by exposure to bacteria and fungi. TREM-1 mediates the acute inflammatory response to microbial products.18 Human tissues infected with bacteria are infiltrated with neutrophils and monocytes that express high levels of TREM-1.18Conversely, TREM-1 is only weakly expressed in samples from patients with noninfectious inflammatory disorders.18 TREM-1 is also shed by the membrane of activated phagocytes and can be found in a soluble form in body fluids.” NEJM 2004; 350:451-458
http://www.nejm.org/doi/full/10.1056/NEJMoa031544#t=article