What Are The Mechanics of a Sulfa Allergy? How Is It Managed?

Recently, a patient had a complex allergic reaction in the form of a rash. I was quite stymied in figuring out what was causing it. Ultimately, I identified her thiazide diuretic as the culprit, specifically the sulfonamides in it. Sure enough, the rash cleared when she discontinued the medication.

A few weeks ago, the patient’s husband asked me, “what is the mechanism of a ‘sulfa allergy’? Does it mean [his wife] can never take a sulfur-based medication again?” I didn’t know the answer to his questions, as I had never given the subject much thought. Thus, I became curious and did some research into the topic.

I discovered that, in most cases, the patient is allergic to sulfonamide medications exclusively. A sulfa allergy doesn’t mean an adverse reaction to everything containing sulfur (such as food products). Sulfur itself is present in most proteins and genetic material. Furthermore, sulfur is an essential component to environmental functions such as the global sulfur cycle.

Conversely, sulfonamides are the molecules only used in medications. There are two classes of sulfonamide medications: antimicrobial and non-antimicrobial. The former is the more allergenic of the two. A sulfa allergy is pretty rare, affecting only 3.4% of the population. 

Patients who are very sensitive to sulfonamides (e.g. HIV patients) have an increased concentration of reactive metabolites. So do people with slower drug metabolism (slow acetylators). Metabolites are the byproducts of the liver’s partial breakdown of sulfonamide. This process can bind proteins in the serum and become “immunogenic”. 

What are the risks of non-antimicrobial sulfonamides? These medications (loop diuretics, sulfonylureas, dapsone) seem less likely to produce an immune system response. If a patient is allergic to sulfonamide antibiotics, the non antibiotics are not necessarily unsafe to use. But in cases where severe reactions occur, use caution. The same holds true for patients allergic to non-antimicrobial sulfonamides: discontinue use if you have a severe reaction.

What Allergen Causes Caterpillar Dermatitis?

Moth and butterfly caterpillars can sometimes cause an allergic reaction known as caterpillar dermatitis (lepidopterism). So after human skin comes into contact with the hairs on a caterpillar’s legs, symptoms begin to show up. While I’ve seen frequent cases of caterpillar dermatitis, I’m still not sure about one key aspect: what specific allergen in the hairs causes the dermatitis?

It turns out that, like porcupines, caterpillars have venomous quills. These spines, called verrucae, contain poisonous fluids, and are nestled beneath the leg hairs. When they penetrate skin, these verrucae introduce the venom into the bloodstream. Hence, an allergic reaction ensues. While the venom is usually not life threatening because of the caterpillar’s small size, it’s still irritating.

Caterpillar dermatitis manifests in hives, itching, and rashes, to name a few. Treatment is relatively simple and non-invasive due to the reaction’s topical nature. If you or your child should develop mild caterpillar dermatitis, use topical corticosteroid cream in addition to an oral antihistamine. As a result, the lesions usually heal within one week without scars.


Caterpillars: MedlinePlus Medical Encyclopedia. U.S National Library of Medicine. https://www.nlm.nih.gov/medlineplus/ency/article/002860.htm. Published July 14, 2015. Accessed July 7, 2016.

Foot NC. Pathology Of The Dermatitis Caused By Megalopyge Opercularis, A Texan Caterpillar. Journal of Experimental Medicine. 1921;35(5):737-753. doi:10.1084/jem.35.5.737.

Müller CSL, Tilgen W, Pföhler C. Caterpillar dermatitis revisited: lepidopterism after contact with oak processionary caterpillar. BMJ Case Reports. http://www.ncbi.nlm.nih.gov/pmc/articles/pmc3082058/. Published April 20, 2011. Accessed July 7, 2016.

Can Cranberries Exacerbate Eczema?

A patient thought that his recent consumption of cranberries exacerbated his eczema. Though allergies to cranberries are highly uncommon, they are associated with immediate type hypersensitivity (hives), but not eczema.

If you are allergic, the best way to treat a cranberry allergy is to avoid cranberry products altogether. Make sure the ingredient list and nutrition facts on all products you consume don’t contain cranberry or traces of cranberry.


Cranberry Allergy Rash. LIVESTRONG.COM. http://www.livestrong.com/article/545907-cranberry-allergy-rash/. Published 2015. Accessed June 29, 2016.

What Percentage of People Are Allergic to Poison Ivy, Sumac, and Oak?

A patient recently asked me what percentage of are allergic to poison ivy. 85% of the American population is allergic to urushiol (Poison Ivy, Sumac, and Oak). Upon first exposure, the skin reaction doesn’t happen for 14 days; subsequent exposures produce reactions in 2 days. The rash lasts an average of three weeks. Here are some tips on dealing with this common summertime rash:

  1. Urushiol  is spread very easily, such on clothing, so be sure to clean thoroughly after being in an area with a high concentration of one or more of these plants.
  2. If you develop a rash, try not to scratch it. Take cool showers or baths to reduce swelling and itchiness. Some topical products you can apply to the affected area are calamine lotion and hydrocortisone cream; both can be purchased over the counter.
  3. The best way you can prevent an allergic contact dermatitis reaction is to wear long pants and sleeves when you are in areas that poison ivy is growing.
  4. If you do manage to come into contact with the plant, make sure that you wash all exposed skin with soap and water as soon as you possibly can. If you rinse off exposed skin within five minutes of coming into contact with urushiol, you can completely remove it.


Poison Plant Identification. , Pictures of Poison Oak & Ivy | Tec Labs. http://www.teclabsinc.com/tips-info/images-video/images/poison-plant-identification/. Accessed June 24, 2016.

Did you know? Poison Ivy, Sumac and Oak  |  American Skin Association. http://www.americanskin.org/resource/poisonivy.php. Accessed June 24, 2016.