In an article about a possible cancer vaccine, Robert Sanders writes that antigens are like red flags, they show the immune system what needs to be attacked, but the kind of flag it is, the way it behaves, or as he writes “the way it’s waved” determines the strength of the immune response.
Concerning bird flu, or avian influenza, it is known that in Influenza A viruses, the body responds to two antigens, both found on the surface of the knobbly sphere-like virus. One is called hemagglutinin (just called H, like H5) and the other is called neuraminidase (N, like N1). H5N1 is just one of many Influenza A viruses.
One interesting antigen may be that of M2e, part of a ion channel protein in all influenza A viruses that is being researched by the company Acambis in the production of a universal flu vaccine.