The immune system in critical illness and injury

During the onset of critical illness or injury, initial attention is focused on supporting vital parameters for patient survival including oxygenation and tissue perfusion. Successful navigation past the primary insult often results in the patient with massive systemic inflammation and multi-organ dysfunction, thus at high risk for progression to organ failure, or secondary infection. It has become increasingly evident that the activation (or deactivation) state of the immune system is highly relevant to the prognosis of every critically ill or injured patient, not just those patients initially presenting with bacterial sepsis. Both the innate immune system, first described by Metchnikov in 1884 with his observations of phagocytosis, and the adaptive immune system are interconnected to sense and respond to invasion of the host by any pathogen. The outcome of these integrated the cellular and humoral responses can range from immediate termination of the microbe at site of invasion, with minimal systemic insult, to profound uncontrolled inflammation and organ damage, to systemic immunoparalysis and death from nosocomial sepsis. A broad overview of host defense is provided below with some discussion of how dysregulation of immune function impacts outcome in the critically ill patient.