Fig. 2

Schematic representation of theories regarding neutrophil subtypes and post-traumatic inflammatory complications. Under homeostatic conditions, there is a homogeneous population of mature neutrophils circulating in the peripheral blood. After trauma, large amounts of immature banded neutrophils enter the circulation. Injury leads to activation of neutrophils and the most activated neutrophils migrate into tissues, leaving less responsive and possibly less competent neutrophils behind in the circulation (the refractory neutrophils). Hypersegmented neutrophils are released into the blood stream after several days post-injury. These cells are known for their decreased bacterial killing after adequate phagocytosis. Therefore, these cells could function as Trojan horses contributing to blood stream infections, especially when present in combination with refractory neutrophils. Also, granulocytic MDSCs with immunosuppressive and immunostimulatory properties are observed after trauma. Persistent expansion of this granulocytic MDSC pool is associated with recurrent infection, prolonged ICU stay, and increased mortality, consistent with the PICS phenotype. MDSC myeloid derived suppressor cells, ICU intensive care unit, PICS persistent inflammation, immunosuppression, and catabolism syndrome