The Complement System in the Setting of Critical Illness—A Narrative Review

The complement system is a key component of innate immunity, known primarily as an immune surveillance mechanism. However, it is also widely known as a modulator of immune responses and inflammation, and its activation has been reported in a wide array of conditions that can lead to admission to the intensive care unit (ICU). Furthermore, various ICU monitoring practices and treatment interventions of the ICU needed to sustain vital organ function may disrupt complement homeostasis. In this review, we will describe in detail the role of the complement system in various critical care settings, with emphasis on major ICU-related conditions such as bacterial and viral sepsis, trauma and burn. Additionally, we will address the potential value of this complex cascade as a prognosis tool and the possible implications for clinical practice as well as its potential as a target for future innovative therapeutic strategies.