Heterosubtypic immunity (HSI) is defined as protective cross-reactive immune responses to lethal infection with influenza A virus of a different serotype than the virus initially encountered, and is thought to be mediated by serotype cross-reactive cytotoxic T lymphocytes (CTL). These CTL recognize conserved epitopes of internal proteins, such as nucleoprotein (NP) or matrix (M) protein shared by influenza A virus subtypes. Despite extensive studies, the precise effector mechanism for HSI remains elusive. For example, our recent studies and those of others reported HSI in T cell-depleted, β2-microglobulin-deficient, and CD8 cell-deficient mice. The role for humoral immune responses in HSI is also unclear. Passive transfer of heterosubtypic immune serum did not provide protection against lethal heterosubtypic challenge, while B cell-deficient mice failed to develop HSI. Our recent findings and those of others now allow us to suggest a two-tiered HSI. Early after heterosubtypic challenge, a number of factors including subtype-specific CTL as well as antibody (Ab) responses and other as yet not well characterized host factors are able to minimize temporarily the virus spread, but are unable to clear the infection. In the later phase, the development of virus-neutralizing (VN) antibodies is important for virus clearance resulting in complete host recovery. © 2001 Elsevier Science B.V.