 The active component of the investigational vaccine is called the protective antigen. When B. anthracis invades the host tissues, the protective antigen activates two other proteins to form anthrax toxin. The symptoms of anthrax are caused by this toxin. Alone, the protective antigen has no known toxicity. The gene for this protein has been isolated and changed specifically to improve its properties as a vaccine-this recombinant protective antigen rPA elicits antibodies that neutralize the anthrax toxin and, thereby, can be predicted to confer protection. 