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Medical Question about antihistamines

  1. Oct 9, 2011 #1

    lisab

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    I have to use antihistamines regularly - usually fexofenadine, but today I used OTC antihistamine eye drops.

    The package insert said the drops were for "pollen, ragweed, grass, and animal hair/dander." I've noticed that most (all?) OTC antihistamines have similar lists of allergens for which you can take the drug.

    WRT antihistamines, does it really matter what the trigger was, once the reaction has started?

    If I'm having an allergic reaction to dust mites, and I take an antihistamine that does not list dust mites, it seems to work just fine.
     
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  3. Oct 10, 2011 #2

    Ryan_m_b

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    I don't think it should make a difference, antihistamines by their very definition are antagonists of histamine and don't directly deal with whatever is causing the response. Perhaps the label is there because whilst antihistamines are OK for those kind of allergies others may cause worse or varied symptoms and require other treatment.
     
  4. Oct 10, 2011 #3
    I'm guessing there are a number of physiological mechanisms that lead to the inlfammatory response (couldn't find anything that said there was a common pathway through histamine). I think that histamine is released from mast cells, leading to inflammation. The mast cells release histamine in response to the binding of an allergen to specific IgE receptors. In which case I'm guessing the antihistamines would work regardless of the allergen? Since, as Ryan said, the antihistamines do not directly interact with the allergen. If there are other pathways from allergen to inflammation, then I'm wrong. Anyone have any more details? I'll be studying immunology soon. This is good preparation!
     
  5. Oct 10, 2011 #4

    bobze

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    Allergic reactions of the kind plaguing you Lisa are http://en.wikipedia.org/wiki/Type_I_hypersensitivity" [Broken].

    First body has to be sensitized to the allergen by B-response helper T cells (Th2s) which do class switching of their Ig production to IgE. On subsequent encounters of the allergen this directly acts on the B cells which cause release of IgE.

    Mast cells and basophils have IgE receptors on their surface which bind the IgE and causes deregulation--Namely of histaminergic granules.

    The release of histamine is what causes the problems in your body, as most cells have histamine receptors--especially vascular endothelial cells and secretory cells. This is the "fast" phase of reaction which causes those yucky "allergy" symptoms.

    In a slower phase, propogated by cytokines, prostoglandins, leukotrines, etc.

    The reaction itself isn't specific for a certain type of allergen, rather is a genetic disposition toward a Th2 mediated IgE response.

    Antihistamines work by blocking histamines action at cell types. First generation antihistamines had the unfortunate (or fortunate if want to go to bed early!) effect of crossing the blood-brain barrier and acting at cells which regulate our sleep patterns--Causing drowsiness. Newer antihistamines have more specificity and generally are much better at acting on peripheral receptors.

    So to directly answer your question--No the allergen itself doesn't matter, its stopping a shared response that the drugs are after.

    In some cases the body can be trained or "desensitized" to the Th2-IgE response through repeated small doses of allergens. Though this doesn't work for everyone and there is obviously a great deal of pharmaceutical research in eliciting a specific Th response instead of the other (Th1 vs Th2).
     
    Last edited by a moderator: May 5, 2017
  6. Oct 11, 2011 #5

    Monique

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    There have been some excellent responses already, I just want to add that for the antihistamines the method of application is important and this does relate to the allergen.

    E.g. generally speaking the eye drops would work well for pollen exposure, an inhalator for animal dander, a lotion or cream for mosquito bites, a tablet for food-related allergies.

    If someone is experiencing a severe peanut allergy it wouldn't make sense to use antihistamine eye drops :wink:
     
  7. Oct 11, 2011 #6

    lisab

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    Thanks, everyone!
     
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