HRs are constitutively activated by binding small amounts of histamine to physiologically maintain their signal-dependent G responses. The accidental triggering of histamine excess above physiological levels, superactivates HRs and their dependent G signals, causing a variety of physiological alterations . Antihistamines reduce their excessive effects either by competing with histamine-binding to HRs (orthosteric) or by inducing conformational HR changes that affect histamine physiological binding (allosteric) . Numerous antihistamine drugs have been used to treat IgE-dependent allergies and anaphylactic shocks reducing histamine-mediated vasodilation and other of their additional dangerous effects.