Anaphylactic reactions involve an immunological mechanism. However, there are subsets of reactions clinically indistinc from anaphylaxis, which occur in a non-IgE-mediated fashion. In these anaphylactoid reactions, certain substances such as morphine and other agents such as radiocontrast media are common triggers. In contrast to IgE-mediated anaphylaxis, prior exposure is generally not required. In non-IgE anaphylaxis, some patients with selective absence of IgA demonstrate anaphylactic symptoms following transfusion of IgA-containing plasma. Physiological changes such as exercise, emotions and overheating  can provoke symptoms  in patients with  cholinergic anaphylactoid  reaction. In this rare condition, an increase in  core body temperature causes a rise in plasma histamine  from mast cells. This disease is an exaggerated form of urticaria. In all cases, no single pathogenic mechanism has been defined, but it is likely that direct mast cell activation accounts for most of these disorders; complement activations has also been reported.