Why am I itching???????

[NbyNW]

Thanks, Possegal!

Can you explain the difference between H1 and H2 antihistamines to a layperson? I have not heard those terms before.

[rubywagon]

I have a great zucchini cake recipe replacer if you're Jonesing for some crab cakes.

[Crankin]

I had hives last winter, at the end of the "wool" season; they appeared mysteriously after a few years of no problems with merino. I've had a history of hives, weird allergies, asthma, so while not unusual, the fact that the wool started to bother me all of a sudden, was weird. Someone here on TE suggested the Zantac, combined with Claritin, and it worked right away.
NY Biker, I feel for you. I have gone through several bouts of what you had and it's scary and annoying.

[Possegal]

Thanks, Possegal!

Can you explain the difference between H1 and H2 antihistamines to a layperson? I have not heard those terms before.

I can try. I taught that topic YEARS ago in a pharmacology class but let's see what I remember. Histamine can bind to a number of different histamine receptors - H1 and H2 are the prominent ones that the drugs tend to target, but there are H3 and H4 receptors as well. Heck, they may have found some others by now. What the histamine causes to happen, depends on what receptor it binds to. And the receptors are more prominent on different tissues. Such as = you have H2 receptors in your GI and when histamine binds to them, gastric acid secretion increases. So if you block that with an H2 antihistamine like Zantac, less gastric acid, less heartburn. THe H1 receptors are what is involved in allergic reactions. THey exist on a lot of different cells including in your airways, where if stimulated by histamine the bronchioles will constrict. Though I think you also have H2 receptors in the airways that if stimulated dilate the bronchioles. So an H2 blocker is not what you would want if you were having a respiratory reaction allergic response. There are a lot more H1 effects like vasodilation (which would be involved in the edema that you can see with allergies), but I'd have to go look them up.

I'll have to dig a bit more to see how blocking H2 treats hives though. It was quite interesting to see that and now I'm curious. I'm sure much more is known about histamine and its receptors since I had any professional reason to be paying close attention.

[NbyNW]

Hey, that's a very helpful explanation! Enough to know what to pay attention to in case of a flare-up.

And good to know that Zantac is possibly helpful in these situations, since I've usually reached for the Benadryl. Nice to have some relief without getting knocked out.

ny biker, hope your symptoms continue to calm down!

[Dianyla]

Histamine can bind to a number of different histamine receptors - H1 and H2 are the prominent ones that the drugs tend to target, but there are H3 and H4 receptors as well. Heck, they may have found some others by now. What the histamine causes to happen, depends on what receptor it binds to. And the receptors are more prominent on different tissues.

Here's a neat map of histamine pathways in the body:

(click for bigger)

[OakLeaf]

Ack, that chart made my head explode!

I was a little surprised that edema (especially facial edema, one of my main symptoms) wasn't on there. Is that related to leukotriene rather than histamine?


ETA: I was really glad to see tachycardia and arrhythmias on that chart! My allergist is well aware of tachycardia in the sense of an increased heart rate that stays within an essentially normal range (although most doctors will roll up their eyes at that one), but I've NEVER had anyone believe that my SVTs correlate with my food allergies. I've been 100% sure of that one for a couple of decades now.

NYB, glad you're feeling better, and hope that avoiding crab takes care of it in the future (although I agree, it would suck if I couldn't eat crab).

[Possegal]

Well since I'm not teaching pharmacology or pathophysiology here, I was aiming to not explode any heads. Then we'd have to put that on the chart too.

I'd put edema in that chart - it is a histamine reaction. As is true of dang near everything in the body, it is probably a combination of things, but in experimental studies you can induce an edematous reaction by infusing histamine.

[Dianyla]

Ack, that chart made my head explode!

I was a little surprised that edema (especially facial edema, one of my main symptoms) wasn't on there. Is that related to leukotriene rather than histamine?

Oops, sowwy!

Edema is actually covered on the chart under endothelial permeability, but that isn't very obvious if you haven't taken biology/anatomy.

[Possegal]

Except that the chart actually shows other clinical signs of histamine's response, so the lack of listing edema, along with flush and uticaria and things like that, is in my opinion, an error. Otherwise you could say that "flush" is already covered under vasodilation. Busy chart for sure, though I've got a way busier one hanging on my door. So I think edema should have been on there.

Similar to this, but the one in poster size has a lot more going on, just can't find a link to one similar. http://www.nature.com/nrn/journal/v4...nrn1197-f3.jpg

[Dianyla]

Except that the chart actually shows other clinical signs of histamine's response, so the lack of listing edema, along with flush and uticaria and things like that, is in my opinion, an error. Otherwise you could say that "flush" is already covered under vasodilation. Busy chart for sure, though I've got a way busier one hanging on my door. So I think edema should have been on there.

True enough!

Similar to this, but the one in poster size has a lot more going on, just can't find a link to one similar. http://www.nature.com/nrn/journal/v4...nrn1197-f3.jpg

Ooh, need to go play with google image search again...