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