Final answer:
Toxoids are protein toxins that have been modified to be non-hazardous while maintaining their antigenic properties for use in vaccines to stimulate an immune response.
Step-by-step explanation:
Protein toxins that have been modified to reduce their hazardous properties without significantly altering their antigenic properties are known as toxoids. This process allows for the use of these substances in vaccines, where they can safely provoke an immune response without causing the disease that their more potent forms would trigger. Toxoids are a form of inactivated toxin used in vaccines such as those for diphtheria and tetanus, which are aimed to stimulate the immune system to produce antibodies against the original toxin, providing immunity.