It's long been known that previous exposure to an influenza subtype - say H1N1 - can leave behind (varying levels) of immunity against future exposures. Those born between 1918 and 1957 - during a time when the H1N1 virus was the only influenza subtype in circulation - fared far better during 1977 return of the `Russian (H1N1) flu' than did children and teenagers born after 1957.
We saw a similar outcome during the 2009 H1N1 pandemic, with those born before 1957 seeing less illness than others, despite the fact that H1N1 had been in circulation (along side H3N2) since 1977.
Today we've research indicating that the first influenza subtype you are exposed to makes the biggest, and most lasting, impression on your immune system. And that the resultant immune response may carry over to other - similar - subtypes.
There are 18 known HA (hemagglutinin) subtypes,which are divided into two major HA groups (1 & 2 see chart below). Depending from which group your first exposure came, you may carry some degree of protection against novel flu viruses of that same group.
In other words, if your first influenza exposure was to H1N1 or H2N2 (group1), you may carry some degree of immunity to the H5 viruses (H5N1, H5N6, etc.). If, however, your first exposure was to H3N2 (group 2), you may carry some protection against H7N9 instead.
And all of this has implications for pandemic planning, prioritization of vaccines (when they are available), and mitigation responses. It also raises some interesting questions on the effect this might have on `universal' vaccines.
For those born before 1968, when H3N2 began its reign, their first flu exposure was almost certainly group 1. Those born between 1968 and 1977, group 2. Those born after 1977, could fall into either category.
It's a fascinating bit of research, and you'll definitely want to follow the link and read it in its entirety, as I've only barely scratched the surface.