Hypersensitivity skin reactions: Clinical

Hypersensitivity skin reactions are due to an exaggerated immune system reaction towards an antigen, and some reactions can be life-threatening.

Hypersensitivity skin reactions include urticaria, erythema multiforme, Staphylococcal scalded skin syndrome, drug reaction with eosinophilia and systemic symptoms, Stevens-Johnson syndrome, and toxic epidermal necrolysis.

Urticaria, also called hives, are slightly raised, well-defined wheals that are 1 millimeter to 10 centimeters in diameter. They’re usually red, blanch with pressure, are extremely itchy, and can pop up anywhere in the body.

The key feature is that these lesions come and go very rapidly - meaning one might appear on the leg as another disappears from the arm.

The reaction involves the epidermis and dermis layers of the skin, and the whole thing typically resolves within 24 hours.

Typically no treatment is needed, but if the itching is really bad, topical cooling moisturizers or oral second-generation histamine H1 blockers can be used like loratadine, desloratadine, fexofenadine, cetirizine, or levocetirizine. If these don’t work, immunomodulatory agents, like cyclosporine or methotrexate can also be used.

Now, if there’s recurrent urticaria, it’s good to try to identify a trigger, so that it can be avoided.

One way is with in vivo skin prick tests, which is where small drops of up to 40 allergens, like pollens, fungi, animal dander, house dust mites, and various foods, are pricked into the skin on the forearm or upper back.

After that, if there are signs of urticaria within about 20 minutes, that implies that the substance is a trigger.