Ineffectiveness of Antihistamines: What to Do?

Ineffectiveness of Antihistamines: What are the Causes?

Antihistamines are drugs that work by blocking the H1 receptors of histamine, a key molecule in allergic reactions. When it binds to its receptors, histamine triggers a cascade of reactions, including an increase in the permeability of blood vessels, which is responsible for the redness and swelling often observed during a skin allergy. By blocking the H1 receptors of histamine, antihistamines therefore help to limit these symptoms and soothe the skin. However, sometimes this action is not sufficient, and this can be for several reasons.

The immune response is not limited to histamine.
Histamine is not the only inflammatory mediator involved in an allergic reaction. Other molecules, such as leukotrienes and prostaglandins, also play a significant role. Therefore, when these other mediators are involved, antihistamines may not be powerful enough to block all the symptoms of the allergy.
Exposure to allergens has increased or the allergy has evolved.
Contrary to a fairly popular misconception, antihistamines do not lose their effectiveness over time. However, there are instances where they may no longer suffice in alleviating allergy symptoms. This could be due to an increase in exposure to the allergen, which could potentially overwhelm the immune system. The production of histamine in the body then becomes too significant for the antihistamines to continue functioning properly and to block the symptoms. It is also possible for the allergy to evolve and for individuals to become sensitive to other allergens. This also results in an accumulation of histamine in the body, too substantial to be effectively managed by antihistamines.
Individual sensitivity does not allow antihistamines to be sufficient.
Some individuals metabolise antihistamines more rapidly than others or are naturally less sensitive to them. This can limit the effectiveness of these medications, even if the dosage is adhered to.

In this context, it is sometimes necessary to explore other options to alleviate the symptoms of skin allergies.

How to respond when antihistamines are not sufficient?

When antihistamines are insufficient, there are additional therapeutic options available to alleviate skin allergies such as urticaria or contact eczema.

Corticosteroids.
Corticosteroids, such as hydrocortisone or betamethasone, reduce local inflammation by limiting the release of pro-inflammatory cytokines. They also act by inhibiting the NF-κB transcription factor through the activation of the IkB gene transcription, thus helping to reduce inflammation. Often prescribed for eczema, the corticosteroids reduce swelling, redness and itching. However, these creams or ointments are intended to be applied for short periods. Indeed, their prolonged use may lead to skin thinning or pigmentation changes.
Calcineurin inhibitors.
In cases where antihistamines prove ineffective following a skin allergy, it is possible to resort to calcineurin inhibitors, such as cyclosporine or tacrolimus. These immunosuppressive drugs block the activation of T lymphocytes, a type of white blood cell involved in the allergic reaction. Furthermore, the inactivation of calcineurin inhibits the dephosphorylation of the transcription factor NF-AT and prevents its translocation into the nucleus, which blocks the release of pro-inflammatory cytokines such as IL-2. Well tolerated, even in the long term, calcineurin inhibitors are nevertheless not recommended for pregnant or breastfeeding women.
A study has examined the benefits of calcineurin inhibitors in cases of eczema. Over a 12-month period, 267 children suffering from atopic dermatitis applied a cream containing 0.03% tacrolimus or a placebo twice daily. Throughout this treatment, the researchers observed that tacrolimus helped to reduce the frequency and intensity of flare-ups, unlike the control. Calcineurin inhibitors thus appear to be interesting substitute medications to antihistamines.
Leukotriene inhibitors.
Leukotriene inhibitors may be a solution when antihistamines do not work. To recall, leukotrienes are mediators of inflammation produced in response to exposure to allergens and are notably metabolites of arachidonic acid. Leukotriene inhibitors, such as montelukast, are primarily used in respiratory allergies like asthma, but they can also be effective in alleviating allergic skin reactions. They work by blocking the leukotriene receptors on inflammatory cells, thus reducing overall inflammation. However, the use of leukotriene inhibitors in skin allergies remains a complementary option, more so explored when the allergy involves both the skin and respiratory systems.
Biological therapy.
Also known as biological immunotherapy, biotherapy can be used to combat severe allergies. This technique involves the use of monoclonal antibodies injected subcutaneously, such as omalizumab or mepolizumab, which specifically target certain molecules involved in the allergic inflammatory response, including IL-2 or the tumour necrosis factor TNF-α. It should be noted that biotherapy is generally not considered as a first-line treatment due to its cost and the need for close medical monitoring.
Desensitisation.
Finally, desensitisation is a process aimed at gradually exposing the body to the offending allergen so that the immune system eventually tolerates it. It is recommended when the allergy becomes debilitating, as can be the case with seasonal allergies to pollen, dust mites or animal hair. It is important to know that the desensitisation process can be quite lengthy and extend over several years. Moreover, allergic reaction-type side effects are common at the beginning of the treatment. However, unlike the other options presented above, desensitisation has the advantage of treating the allergy and not just its symptoms.

The use of anti-mite covers, hypoallergenic cleaning products, and limiting exposure to allergens as much as possible are simple actions that should not be underestimated in the fight against skin allergies.