The majority of histamine allergies are on H1 and H2. H3 and H4 are pretty rare. H1 are where most allergic reactions take place while H2 is the gastric stuff. I imagine if you're taking H1, most of your other histamine allergies will also be impacted. H3 is mostly found in the brain and H4 is kind of special. H4 was found while combing through the human genome and it seems to affect the shape of some white blood cells and the movement of mast cells. H4 is not well understood as it was only discovered in 2000. H4 receptor antagonists are being studied for the potential treatment of asthma.

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