The Collaborative International Dictionary of English v.0.48:

anaphylaxis \an`a*phy*lax"is\ n.
   ([a^]n`[.a]*f[imac]*l[a^]k"s[i^]s or
   [a^]n`[.a]*f[i^]*l[a^]k"s[i^]s)
   1. [Gr. 'ana` back, way from + fy`laxis security,
      protection.]

   1. (Med.) Hypersensitivity (to a foreign antigen) resulting
      from prior contact with the same or a related substance.
      [WordNet 1.5 + AS]

   2. (Med.) An immediate but transient systemic reaction to an
      antigenic substance to which a person has become
      hypersensitized; it is characterized by the contraction of
      smooth muscle and capillary dilation. --[Stedman]
      [PJC + AS]

   Note: The cascade of events is triggered by binding of the
         foreign antigen to immunoglobulin E and release of
         mediators such as histamine, bradykinin, serotonin, and
         slow-reacting substance from mast cells. It involves
         more than one organ system. The most commonly involved
         organs are skin, cardiovascular, respiratory and
         gastrointestinal systems. Anaphylaxis is manifested by
         a range of signs and symptoms including pruritus,
         urticaria, itching, dyspnea, abdominal pain and
         cardiovascular collapse (shock). A severe form of the
         reaction is called anaphylactic shock. --[Stedman]
         [PJC + AS]

   3. (Med) Anaphylactic shock.
      [AS]

The Collaborative International Dictionary of English v.0.48:

anaphylactic shock \anaphylactic shock\ n.
   ([a^]n`[.a]*f[imac]*l[a^]k"t[i^]k sh[o^]k`) (Med.)
   Shock resulting from anaphylaxis[2]. It is a severe form of
   physiological shock, often having a fatal outcome, and is
   caused by an extreme immunological reaction to antigens. It
   is characterized by smooth muscle contraction and capillary
   dilation throughout the body, initiated by antibodies of the
   IgE class. It is usually seen when an antigen to which a
   person has become hypersensitized is injected intravenously
   or subcutaneously. It is also called generalyzed
   anaphylaxis or systemic anaphylaxis, and sometimes
   anaphylaxis. --[Stedman]
   [PJC + AS]

WordNet (r) 3.0 (2006):

anaphylaxis
    n 1: hypersensitivity reaction to the ingestion or injection of
         a substance (a protein or drug) resulting from prior
         contact with a substance

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "anaphylaxis":
   allergy, considerateness, delicacy, empathy, exquisiteness,
   fineness, hyperesthesia, hyperpathia, hypersensitivity,
   identification, irritability, nervousness, oversensibility,
   oversensitiveness, overtenderness, passibility, perceptiveness,
   perceptivity, photophobia, prickliness, responsiveness,
   sensitiveness, sensitivity, sensitization, soreness,
   supersensitivity, sympathy, tact, tactfulness, tenderness,
   tetchiness, thin skin, ticklishness, touchiness