Why am I itching???????

[ny biker]

So, just got back from the allergist. The conclusion in summary: we don't really know what caused the reaction.

He thought the crab cakes sounded like the most likely cause, due to the timing of when I ate them and when the symptoms began. Then he tested me for allergies to a bunch of different foods but none yielded a positive result. However, he told me that the tests for food allergies are less reliable than tests for things like mold and dander, and in fact the test for cashews was also negative even though I've had a clear reaction to them in the past. Also it's possible that different types of crabs have slight differences in protein structure and one could cause an allergy while the other doesn't.

So he recommended that I avoid crab and other shellfish for now. In a few months, I can try a nibble of someone else's crab cake. If I have no reaction, I can try a larger nibble at another time, and if that's okay I can have a whole bite. If there's no reaction to that, I can have a whole crab cake and I'll probably be okay.

In the meantime, he prefers Zantac as an antihistimine for this type of reaction, and he recommends I keep some on hand (and I can keep the Claritin around also -- no harm to use both) in case I have problems in the future. He's not a big fan of Benedryl unless I'm having trouble sleeping due to the itch (which definitely was the case with this recent attack).

I was prepared for negative test results, so I'm not really disappointed or surprised. Overall I liked the doctor a lot. This was my first visit with him. He's an older man (called me "young lady" twice, which is something I rarely hear these days ) but from talking to him I could tell he keeps on top of current research.

Some other interesting things he mentioned -- a large number of people who get hives have them due to an autoimmune response, not because of a specific allergy trigger. Food allergies are more likely to manifest within ~12 hours of eating the food, so he wasn't particularly interested in anything I'd had prior to the day the attack happened (the symptoms began around 7 pm). It's possible that the symptoms might return in a few days, because I'm still tapering off the prednisone and just as there is a 1-2 day lag for that drug to take effect, there is a 1-2 day lag for it to completely clear your body. So I should have Zantac handy for the next week or two, just in case.

I do love crab cakes and will miss eating them.

BTW I did not mention the possibility of pseudoallergy/histamine intolerance to the doctor, mostly because there were so many other things to discuss while I was there and I would like to focus on the crab thing for now. But I'm keeping it in the back of my mind for the future. For example in the week before this happened I changed two bike tires (latex exposure) and ate a tuna sandwich with avocado which has the same allergen in it as latex. In the future I'll try to be aware of those kinds of multiple exposures that might cause a cumulative affect. I did have the same sandwich with avocado yesterday for lunch, and have not had any ill effects.

I have to say, allergies suck, but on another level they are kind of fascinating.

[kmehrzad]

Hi NY Biker,

I've had hives pop up for no reason at various times throughout the year. After undergoing allergy testing and coming up negative, the drs. were unable to give me a reason for the condition. We explored the stress issue and that wasn't relevant. In any case, I carry (or carried) Loratdine around with me and would take one whenever I felt the itching sensation, that took care of the symptoms.

[redrhodie]

I've had reaction to shellfish, but only if I ride on the day I eat it. Otherwise, I don't react. Look up "exercise-induced anaphylaxis" if you rode that day.

[NbyNW]

Interesting. I've always thought of Zantac as an ulcer medication (a proton pump inhibitor? my brain is rusty), rather than an antihistamine.

I may have to give that a try the next time I have a mysterious reaction to something.

[Possegal]

Interesting. I've always thought of Zantac as an ulcer medication (a proton pump inhibitor? my brain is rusty), rather than an antihistamine.

It is a heartburn/ulcer med but it is an H2 antihistamine, not a PPI. Things like Benadryl and Claritin are H1 antihistamines. I think the use of the H2 blockers for allergies is a somewhat new use (not an approved indication that I know of, but I'll look and see.) From what I read it looks like it is usually only recommended when the allergic reaction is hives. Again, just a quick look at this point. I'm a Dr of Pharmacology and someone who, as my mother always said came out of the womb with allergies, and yet I hadn't heard of docs using ranitidine for allergies.

[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