Everybody knows what allergies are, but what causes them? What drugs do we have available to address them? As we learned in the immunology series, allergies are symptoms of an immune response to a perceived threat. Histamine is a molecule in the body that orchestrates many aspects of this response, and antihistamines are a class of drugs whose purpose it is to neutralize the effects of histamine in the body. Many of these are over-the-counter drugs which should be familiar to us, like Benadryl, Claritin, Zyrtec, and Allegra. How do these drugs work on the molecular level? Let's find out!