The toxoid gives long term immunity, but it takes 10 days to 2 weeks for the body to start building the immunity. The initial
Injection is followed 3-4 weeks later and is supposed to give immunity for about a year. It may last longer, some people do not give the booster yearly.
The ANTI toxin is for immediate immunity. (To prevent or hopefully cure clostridial diseases caused by over eating, getting into chicken feed, etc.) But it wears off in a couple weeks and you need to give the toxoid.