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):

anaphylactic shock
    n 1: a severe and rapid and sometimes fatal hypersensitivity
         reaction to a substance (especially a vaccine or penicillin
         or shellfish or insect venom) to which the organism has
         become sensitized by previous exposure