Adacel contains pertussis antigens, diphtheria toxoid, and tetanus toxoid. Toxoids are bacterial toxins that have been detoxified. Although the toxoids will not cause the actual disease, they stimulate the immune system to produce antibodies to the toxin.
"Antigen" is the generic term for substances that cause the body to produce an immune response and to produce antibodies. Adacel contains a few different pertussis antigens which have been detoxified. Much like the toxoids, the detoxified antigens will not cause the actual disease but can stimulate an immune response.
Basically, the antigens and toxoids in Adacel "trick" the body into thinking it has been exposed to diphtheria, tetanus, and pertussis. The body produces antibodies that will help fight these diseases if future exposure occurs.
Adacel is approved for children as young as 11 years old. Other similar vaccines (the DTaP vaccines) are approved for younger children.
Sometimes healthcare providers may recommend a medicine "off-label" for something other than the approved uses. At this time, however, there are no universally accepted off-label uses for Adacel.