Antigen
Antigens are "targeted" by antibodies. Each antibody is specifically produced by the immune system to match an antigen after cells in the immune system come into "contact" with it; this allows a precise identification or matching of the antigen and the initiation of a tailored response. The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation in a region of the antibody; because of this, many different antibodies are produced, each able to bind a different antigen while sharing the same basic structure. In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react and bind more than one antigen.
Bette Korber
Several of the major variations are included in each molecule of protein, thus producing a variant protein antigen that probably does not exist in the wild virus population but should cross-react with variants that do exist.  Korber has taken two different approaches to designing such antigens.  Her group has developed a computer algorithm to choose epitopes to combine into a mosaic molecule for the mosaic antigens. In 2009, she described a designed mosaic protein this way: "People didn't know if it would fold properly, if it would be antigenic, or if it would have the same sites that recognized by killer T cells". They found that the newly designed antigens did fold properly and acted as a strong antigen, and were recognized by the cytotoxic T cells (killer cells). Also, Korber and her collaborators have developed a graphical analysis called Epigraph that can generate promising antigens with a mixture of epitopes. Korber explains that the approach of designing a protein via computer, combining bits of known proteins that provoke immune responses, had never been tried. She says, "Even after it worked, it was hard to convince people that this novel thing could be a vaccine because it hadn't been done before".