The B subunits of A-B toxins are used as vaccine components without any chemical treatment. Why is it safe to use B subunits as they are? What would happen if you injected the A chain of DT into your...

The B subunits of A-B toxins are used as vaccine components without any chemical treatment. Why is it safe to use B subunits as they are?

What would happen if you injected the A chain of DT into your bloodstream? Could the A chain be used as a vaccine component?