Hmm I'll try remember.
Basically if the virus mutates so much, then the protein antigens on its surface will also mutate. The human immune response is initially triggered by antigens, and immunity is provided by B and T cells that recognise their specific antigens.
Normally, the influenza antigens would be recognised immediately by the B and T lymphocytes, and the immune response is activated immediately to destroy the pathogens, so that the individual does not fall ill. However if the antigens continue to be different because of the mutation, then the individual will be unable to develop effective immunity as the virus will continue to be recognised as a new antigen, without memory cells corresponding to it in the blood. Hence they will most likely fall ill, as the body has to work to develop new antibodies.
Basically yeah, it would be hard for humans to develop immunity against the flu as the antigens would keep changing. Immunity would only be effective if the individual were to come into contact with the same pathogen, that had the same unmutated antigens.
It went something like that. But made sense. With detail.