The Collaborative International Dictionary of English v.0.48:

allergy \allergy\ n.
   1. hypersensitivity to a particular substance. "an allergy to
      bee venom so severe that a second sting may be fatal"
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

allergy
    n 1: hypersensitivity reaction to a particular allergen;
         symptoms can vary greatly in intensity [syn: allergy,
         allergic reaction]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "allergy":
   abhorrence, abnormality, abomination, acute disease, affection,
   affliction, ailment, allergen, allergic disease, allergic disorder,
   anaphylaxis, antagonism, antipathy, asthma, atrophy, aversion,
   bacterial disease, birth defect, blight, cardiovascular disease,
   chronic disease, circulatory disease, cold sweat, complaint,
   complication, condition, congenital defect, conjunctivitis,
   considerateness, cosmetic dermatitis, creeping flesh, defect,
   deficiency disease, deformity, degenerative disease, delicacy,
   disability, disease, disgust, disorder, distemper, eczema, empathy,
   endemic, endemic disease, endocrine disease, enmity,
   epidemic disease, exquisiteness, fineness, functional disease,
   fungus disease, gastrointestinal disease, genetic disease,
   handicap, hate, hatred, hay fever, hereditary disease, hives,
   horror, hostility, hyperesthesia, hyperpathia, hypersensitivity,
   iatrogenic disease, identification, illness, indisposition,
   infectious disease, infirmity, irritability, loathing, malady,
   malaise, morbidity, morbus, mortal horror, muscular disease,
   nausea, nervousness, neurological disease, nutritional disease,
   occupational disease, organic disease, oversensibility,
   oversensitiveness, overtenderness, pandemic disease, passibility,
   pathological condition, pathology, perceptiveness, perceptivity,
   photophobia, plant disease, pollinosis, prickliness,
   protozoan disease, psychosomatic disease, rejection, repugnance,
   repulsion, respiratory disease, responsiveness, revulsion,
   rockiness, rose cold, secondary disease, seediness, sensitiveness,
   sensitivity, sensitization, shuddering, sickishness, sickness,
   signs, soreness, supersensitivity, sympathy, symptomatology,
   symptomology, symptoms, syndrome, tact, tactfulness, tenderness,
   tetchiness, the pip, thin skin, ticklishness, touchiness,
   urogenital disease, urticaria, virus disease, wasting disease,
   worm disease