The Collaborative International Dictionary of English v.0.48:

Antitoxin \An`ti*tox"in\, Antitoxine \An`ti*tox"ine\, n. [Pref.
   anti- + toxin.]
   A substance (sometimes the product of a specific
   micro-organism and sometimes naturally present in the blood
   or tissues of an animal), capable of producing immunity from
   certain diseases, or of counteracting the poisonous effects
   of pathogenic bacteria.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antitoxin
    n 1: an antibody that can neutralize a specific toxin

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "antitoxin":
   Rh factor, agglutinin, allergen, anaphylactin, antiantibody,
   antibody, antigen, antiserum, antitoxic serum, antivenin, bang,
   booster, booster shot, fix, hit, hypodermic, hypodermic injection,
   incomplete antibody, injection, inoculation, interferon,
   jet injection, mainlining, narcotic injection, precipitin, serum,
   shooting up, shot, skin-popping, vaccination, vaccine