Abstract
Well understood are the adaptive and dramatic neutralizing homosubtypic antibody responses to hypervariable, immunodominant sites of the hemagglutinin (HA) and neuraminidase (NA) of individual influenza strains. These define influenza subtypes and vaccines modelled upon their HA and NA antigens provide seasonal neutralizing antibody protection against subsequent exposure to the strain and its close relatives, but give little if any protection against antigenically drifted or shifted strains. Contrasting to this is a different form of acquired antibody response, called heterosubtypic immunity. This provides a more seasoned adaptive antibody response to immune-recessive epitopes that are highly-conserved amongst strains. Although, such responses are of lower individual amplitudes than seasonal mechanisms they are active across influenza subtypes, and may give pre-emptive protection against new strains yet to emerge. Heterosubtypic immunities have been well studied in animals, but surprisingly there is minimal evidence for this type of antibody immunity in humans. Thus championed is the notion that seasoned humoral responses can through repeated exposure to sites widely conserved across different strains, cumulatively provide humans with a level of broad protection against emergent novel strains, such as H5N1, that is not afforded by seasonal humoral responses.