Infection with one serotype of the influenza A virus confers a certain level of heterosubtypic immunity (HSI) to infection with influenza A virus of a different serotype. Effective HSI is thus predicated on prior sensitization of the host, implicating that adaptive immunity plays a vital role. It had been proposed that HSI is mediated by serotype cross-reactive cytotoxic T lymphocytes (CTL), but more recent studies suggest a principal role for antibodies (Abs) in HSI. Our results from microarray analysis of gene expression suggest a role for complement (C) in HSI. Accordingly, we found that complement protein C3 is required for induction of antigen-specific Ab responses and subsequent complete HSI. In addition, complement per se may play a role in effector mechanisms in HSI. These findings suggest that both adaptive and innate immunity are required for protective HSI. © 2004, Elsevier B.V.