First we must describe how immunity works. Immunity works by recognition of the antigen by macrophages or dendritic cells (antigen presenting cells) then presenting it to T cells and T cells will work to destroy the organisms with the specific antigen. B cells can also recognize antigens then secrete antibodies to combat organisms with the specific antigen. In the case of flu, or influenza, the genetic material of the virus mutates so often that the antibodies produced by the body against a certain strain of influenza will not work after some time because of the mutation of the virus.