The Complement System and Its Role in Eosinophilic Inflammation in Respiratory Diseases

The complement system is a key link between innate and adaptive immunity, contributing to pathogen elimination, immune regulation, and tissue homeostasis. Its activation is not only crucial in infections, such as COVID-19, but also plays a major role in the pathomechanism of several non-infectious respiratory diseases, such as asthma, COPD, sarcoidosis and lung cancer. Complement components can modulate the quality of the adaptive immune responses, including through the regulation of T2 immunity and eosinophilic inflammation, thereby linking natural defense to complex immune processes. In recent years, it has become increasingly clear that dysregulated complement activity contributes to inflammation, thrombosis and tissue damage in a wide range of respiratory diseases. The study of the various components of this cascade system may therefore be promising from both a diagnostic and therapeutic point of view. Some of its components may serve as biomarkers for distinguishing between different phenotypes of certain lung diseases, while their targeted inhibition or modulation may open the way towards new treatment options. A better understanding of the complement system's integrative and regulatory role not only allows for a deeper insight into immunological interactions but may also bring us closer to phenotype-oriented, immunology-based pulmonology, which may have real clinical benefits in the future.