As soluble recognition molecules of innate immunity, C1q and MBL are able to bind directly to various viruses, including retroviruses and influenza viruses. Interaction of C1q with retroviruses and certain infected cells was shown to involve the globular region of C1q and viral envelope glycoproteins, such as p15E of MuLV, gp41 and gp120 of HIV-1, gp21 of HTLV-1. C1q binding was found to trigger antibody-independent activation of the classical pathway of complement, but did not lead to virus destruction and had even an adverse effect on infection in humans, because of subversion of the complement system by the virus. Binding of MBL or of the pulmonary collectin SP-D to influenza A virus was shown to involve the carbohydrate recognition domain of the molecule and high-mannose oligosaccharides of the viral proteins haemagglutinin and neuraminidase. These interactions lead to virus inactivation, are independent of complement activation and are influenced by the oligomerization state of the collectin.