What does H2 stand for?
H2 stands for histamine type 2
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Examples of H1 receptor antagonists include diphenhydramine (Benadryl) and loratadine (Claritin). Interestingly, discovery of new histamine receptors largely followed the findings that the H1 antagonists did not block all actions of histamine. For example, H1 receptor antagonists do not effect secretion of gastric acid because that response is due to binding of histamine to H2 type.
Histamine's second type of allergic response is one of the major causes for asthma. In response to an allergen (a substance that triggers an allergic reaction), histamine, along with other chemicals, causes the contraction of smooth muscle (Schmidt et al 1999). Consequently, the muscles surrounding the airways constrict causing shortness of breath and possibly complete trachial-closure, an obviously life-threatening condition.