IFITM3 and infection: From our findings we assume two putative and presumably additive mechanisms for IFITM3 mediated antiviral action: (i) The first contact with IAV particles after an endocytic uptake in (early) endosomes (role of recycling endosomes is discussed below) triggers the continuous recruitment of pre-existing IFITM3 proteins to IAV containing compartments as a fast antiviral defense in the initial phase of infection.