The present invention mainly provides a fusion antigen specific for a target cell comprising a ligand moiety which is capable of reacting, recognizing or binding to the receptors on the target cell, a Pseudomonas exotoxin A translocation domain II, an antigenic moiety, and a carboxyl terminal moiety which permits combination of the fusion antigen to the endoplasmic reticulum (ER) membrane of the target cell. A method of immunizing an animal using the fusion antigen is also provided.