Antibodies against the external proteins of influenza can prevent the virus from infecting cells and either prevent infection or limit the spread of infection. However the surface proteins are highly variable and there is little antibody cross-reactivity between variants. Once a cell has been infected with the virus, it is then vulnerable to T cell attack resulting in the destruction of infected cells so that no more virus can be produced and the infection is controlled. There is evidence from clinical trials of influenza challenge, and animal models that T cell responses can protect in the absence of antibodies. Additionally, since T cells can recognise the highly conserved internal proteins of influenza, cross-subtype protection can be achieved.