Invasive aspergillosis shows a high mortality rate particularly in immunocompromised patients. Perpetually increasing numbers of affected patients highlight the importance of a clearer understanding of interactions between innate immunity and fungi. Innate immunity is considered to be the most significant host defence against invasive fungal infections. Complement represents a crucial part of this first line defence and comprises direct effects against invading pathogens as well as bridging functions to other parts of the immune network. However, despite the potency of complement to attack foreign pathogens, the prevalence of invasive fungal infections is increasing. Two possible reasons may explain that phenomenon: First, complement activation might be insufficient for an effective antifungal defence in risk patients (due to, e.g., low complement levels, poor recognition of fungal surface, or missing interplay with other immune elements in immunocompromised patients). On the other hand, fungi may have developed evasion strategies to avoid recognition and/or eradication by complement. In this review, we summarize the most important interactions between Aspergillus and the complement system. We describe the various ways of complement activation by Aspergillus and the antifungal effects of the system, and also show proven and probable mechanisms of Aspergillus for complement evasion. 1. Aspergillus Evokes Invasive Infections in Immunocompromised Individuals Aspergillus species are ascomycetes that are classified in the form subdivision Deuteromycotina, as many of them do not show a sexual reproductive phase [1]. Generally, they are common ubiquitous saprophytes in soil and on dead organic substrates. Being classic opportunistic pathogens, invasive infections by Aspergillus species almost exclusively develop in immunocompromised patients, while localized infections and allergic bronchopulmonary aspergillosis occur in individuals without immunosuppression. Generally, the species Aspergillus fumigatus represents the most common inducer of invasive and allergic manifestations, followed by A. terreus, A. flavus, and A. niger [1, 2]. Invasive aspergillosis (IA) considerably contributes to the morbidity and mortality among immunocompromised individuals, including patients with haematological malignancies, recipients of haematological stem cell and solid organ transplants, AIDS patients, and patients treated with immunosuppressive regimens due to autoimmune diseases [3]. The most important single risk factor is prolonged and profound neutropenia (<500
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