Because your hands and arms are so busy getting you around in the world, carrying your burdens and helping you work, their skin is a primary contact point for allergens. In addition, they may be the breakout spots for allergies to things you've ingested by eating or breathing. Dermatologists and allergists suggest you try symptomatic treatment first, but go for help before you resort to scratching. That only opens your skin to infection on top of the allergy.
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Hives, formally known as urticaria, are swellings on the skin, sometimes with blistery pustules on top, that can come up almost anywhere on the body, including the arms and especially the soft insides of elbows and upper arms. They are not always allergic, but can also be signs of stress or illness. As allergic symptoms, hives can also point either to contact with an allergen or to ingested allergens, including medicines such as penicillin. Hives on your arms could alert you to a food or contact allergy with the potential to turn life-threatening, or they could warn you to search out the source of mold or another inhaled allergen in your home.
Skin can become inflamed when you encounter an allergenic oil or dust, such as the oil of poison ivy or poison oak. The reaction can range from a slightly itchy redness to large and spreading blisters, depending on your exposure and sensitivity. Try to identify what you've contacted, and wash the area and any clothes that may carry the allergen. You also could be allergic to something in the detergents or skin-care products you use. For example, chamomile and calendula are common botanical oils that can cause contact dermatitis in people who sneeze at ragweed pollen.
Also known as atopic dermatitis, eczema is more common on the elbows than elsewhere on adults' arms. It can reflect an allergy to something in the air such as pet dander or house dust, or to something that's been rubbed over the elbows, such as wool or soaps. It generally appears as skin that's extremely dry, to the point of cracking and bleeding, and may also itch. You can ease the symptoms with hypoallergenic creams or with topical steroids.
Almost any allergy or sensitivity can show up on the arms. The good thing is that you're likely to notice it on yourself or a loved one. Pay attention to the speed with which any rash or reaction spreads. For example, a severe latex allergy can start as swollen, red hands that have been in gloves or just spots on the skin where it was touched. It can progress rapidly, however, to anaphylactic shock that requires immediate medical care. A sensitivity to adhesives such as you find in bandages is less usually urgent but can be upsetting. Switch to paper tape or self-adhering tape.