In the course of influenza, deposition of the complement component 3 (C3) and MAC in the lungs was observed just a few days after infection, as well as increased expression of C5aR1 in the lungs, especially on bronchial epithelial cells and inflammatory cells, and an increased concentration of C5a anaphylatoxin in plasma and BALF [97,98]. Here, C5 is linked to infection.