Our bodies are good at generating extremely specific antibodies, targeting a single pathogen among a dizzying mix of harmless bacteria and the proteins made by our own cells.
In recent years, however, it has become apparent that the immune system sometimes gets wildly lucky by generating a single antibody that can neutralize a huge range of viruses.
Mass production of these antibodies might provide a useful therapy, and the hope is that we can incorporate what they tell us into the design of future vaccines for these pathogens.
But that creates a strong evolutionary selection for mutations that change these features.
Some of those are even luckier still, and they stick to part of a protein that's utterly essential to the pathogen and block its function.
In the case of the flu virus, broadly neutralizing antibodies have been discovered, and these tend to bind to the virus' hemagglutinin protein (the H in the flu's HA nomenclature).