WordNet (r) 3.0 (2006):

antigen
    n 1: any substance (as a toxin or enzyme) that stimulates an
         immune response in the body (especially the production of
         antibodies)

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "antigen":
   Rh factor, Rh-negative, Rh-positive, Rh-type, Rhesus factor,
   acquired immunity, active immunity, agglutinin, allergen,
   anaphylactin, antiantibody, antibody, antiserum, antitoxic serum,
   antitoxin, antivenin, arterial blood, artificial immunity, blood,
   blood bank, blood cell, blood count, blood donor,
   blood donor center, blood group, blood grouping, blood picture,
   blood platelet, blood pressure, blood serum, blood substitute,
   bloodmobile, bloodstream, circulation, clinical dextran,
   congenital immunity, dextran, erythrocyte, familial immunity,
   globulin, gore, grume, hematics, hematologist, hematology,
   hematoscope, hematoscopy, hemocyte, hemoglobin, hemometer, humor,
   ichor, immunity, immunization, incomplete antibody,
   inherent immunity, inherited immunity, interferon, isoantibody,
   leukocyte, lifeblood, natural immunity, neutrophil,
   nonspecific immunity, nonsusceptibility to disease,
   opsonic immunity, opsonin, passive immunity, phagocyte,
   phagocytic immunity, plasma, plasma substitute, precipitin,
   racial immunity, red corpuscle, resistance, serum,
   specific immunity, toxin-antitoxin immunity, type O, venous blood,
   white corpuscle