Explain how an adult develops immunity to the toxin.

Adults can develop immunity to a toxin via being exposed to the toxin, whether this is by infection or vaccination. Since this toxin will be recognised as foreign by the body, this will stimulate an immune response, which will lead to the production of B lymphocytes. These will produce antibodies which will bind to the toxin so it can be removed from the body and not harm the host. Once the risk from the toxin has been neutralised, memory lymphocytes will remain in the circulation to protect against subsequent infection by the same toxin.