WordNet (r) 3.0 (2006):

allergen
    n 1: any substance that can cause an allergy

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "allergen":
   Rh factor, agglutinin, allergic disorder, allergy, anaphylactin,
   antiantibody, antibody, antigen, antiserum, antitoxic serum,
   antitoxin, antivenin, asthma, conjunctivitis, cosmetic dermatitis,
   eczema, hay fever, hives, incomplete antibody, interferon,
   pollinosis, precipitin, rose cold, serum, urticaria