Activity can be quantitated by LD 50 . Thus toxins are toxic. The lethality of toxins varies with animal species and can provide a basis for species specificity. Generally secreted and act at a distance. Most function as either enzymes, pore-formers, or superantigens.
Toxoids are the basis of some vaccines.
Classes A. Enzymes 1. Bind via the B subunit to host cell receptors and are internalized, activated and delivered to the target. Proteolytic toxins, e.g. tetanus and botulinum neurotoxins. Both toxins are zinc metaloproteases and share considerable homology. These toxins are used at very low doses therapeutically to treat nerve disorders, and even to treat wrinkles.