Determination of Serum Antibody to Bordetella pertussis Adenylate Cyclase Toxin in Vaccinated and Unvaccinated Children and in Children and Adults with Pertussis (Cherry, 2004, Clin Infect Dis.)
When the immune system meets a pathogen for the first time, it elicits an immune response. The second time the same or a similar pathogen is encountered, the immune system uses the same immune response, even if another (new) response would have been more effective. This phenomenon is called "original antigenic sin". In the case of whooping cough, after the pertussis bacterium settles in the airways, one of the toxins that it secretes, namely adenylate cyclase toxin (ACT), deceives the immune system so that it doesn´t recognise pertussis as a pathogen. After two weeks, however, the immune system realizes that it was deceived, and begins to fight against the pertussis infection. The next time the immune system will come across ACT, it will not be deceived and will immediately suppress the pathogen. Thus, an individual will not become infected a second time. However, since there is no ACT in the vaccine, the immune system of the vaccinated individual does not yet know how to react to it, and the vaccinated individual becomes infected with whooping cough. Now, as a result of original antigenic sin, the immune system in this case will never learn how to respond effectively to pertussis.
Moreover, the more doses an individual receives, the greater the effect of original antigenic sin. This is because with each dose, the immune system will produce more and more specific B-cells. These cells end up competing with naive B-cells that could have adapted and responded more efficiently to a slightly altered pathogen.
Thus, since the vaccine pathogen and the natural pathogen differ from each other, the immune response to the real pertussis bacterium in an unvaccinated individual will be much more effective than the immune response in a vaccinated individual. In this way, an unvaccinated individual will suffer pertussis once and only once, while a vaccinated individual will have ineffective response to the bacterium for the rest of his or her life.