In addition to its natural biological innate immunity, the human body has the ability to develop extremely powerful specific immunity against individual invading agents such as lethal bacteria, viruses, toxins, and even foreign tissues from other animals. This ability is called acquired or adaptive immunity. Acquired immunity is developed by a special immune system that forms antibodies and/or activated lymphocytes that attack and destroy the specific invading organism or toxin.

Acquired immunity usually is able to give a powerful degree of protection to the body. For instance, certain toxins, such as the paralytic botulinum toxin or the tetanizing toxin of tetanus, can be protected against in doses as high as 100,000 times the amount that would be lethal without immunity. It is for this reason that the treatment process known as immunization is so important in protecting human beings against diseases and toxins.