An antitoxin is a pharmaceutical preparation of antibodies used to treat a particular toxin.
Although many toxins (most notably alcohol and acetaminophen) can be processed through the liver to transform them into harmless compounds, the immune system can also play a role in eliminating toxins. In the same way that it fights off infection, the body can create antibodies that bind to and eliminate toxins, most notably the toxins related to diphtheria and botulism.
However, this ability to create antibodies takes time (generally somewhere between 1 and 2 weeks). However, the tolerance, once gained, is maintained by the patient. As such, as with blood serum used to treat certain types of infection, blood products can be processed to create antitoxins as well. In addition, since the antibodies are not human specific, non-humans (typically horses) can be used to create antitoxin serums in large quantity. However, the use of human serum is less likely to result in an adverse reaction.