Hives are a specific type of rash that is often very itchy. They can be small bumps or look something like the flat raised welt caused by a mosquito bite. They can get wider or combine into an expansive hive “area” as an allergic reaction progresses. Histamine release causes the development of hives. Since scratching releases additional histamines, scratching can lead to more hives.

Hives are unusual because they may be caused by allergens, as well as by heat or cold exposure, physical pressure or excessive sweating.

Treatment for hives is the same regardless of their cause. Antihistamines are the mainstay, and corticosteroids are used to block the immune activity in stubborn hives. If you have recurring hives and you don't know why, see your health-care provider. In rare cases, hives have been associated not with allergy but with infection, thyroid problems, lupus, hepatitis and diseases affecting blood vessels. (In these instances, hives are rarely the only symptom.)

Eczema refers to an inflamed skin rash characterized by thickened, reddened skin, sometimes with dry peeling areas on the skin surface (called “scale”). Eczema may also take the form of inflamed red patches that sometimes seep clear fluid It may or may not be caused by an allergy.

It is important to know if your eczema is related to allergy because this affects how you treat the rash. Antihistamines are the mainstay of treatment for allergy-related rashes; these medications not only to control the itching but also prevent scratching. Scratching itself can actually make the rash worse by releasing more histamine and stimulating inflammation. Often, allergy-related eczema also requires topical corticosteroids to calm down inflammation and the immune system.

“Atopic dermatitis” and “atopy” are terms used when an eczema rash is blamed on allergy. Commonly, a specific trigger is not identified but the person with eczema might have other symptoms that suggest allergy, such as allergic rhinitis. In some children with atopic dermatitis, itchy red or purple patches appear on the face, neck or arms. Children with atopic dermatitis tend to have a family history of allergies or eczema. They may have food allergies to eggs, milk, peanuts, soy, wheat or fish, or they may have allergic rhinitis or asthma.

Atopic dermatitis usually becomes milder with age and, especially in adulthood, may become limited to one area of the body, such as the hands, neck or creases inside an elbow or knee. Some people will have atopic dermatitis as children and no rash as adults.

Dry or cracked skin can cause a person with eczema to have a more active rash. It is possible that healthy skin helps to prevent eczema, by preventing exposure to allergens or skin irritants. Preventing dry skin is one of the most important ways to control eczema. It is best to avoid very hot baths and drying soaps or lotions (which may contain alcohol and cause natural skin moisture to evaporate). Lubricating creams (that are not water or alcohol-based) or ointments are quite helpful.

Very frequently, eczema (including atopic dermatitis) requires topical corticosteroids to calm down inflammation. Other potent antiinflammatory medicines can be prescribed for severe cases. Antihistamines may be useful for reducing itching, but they have not been proven to cause improvement in an eczema rash.

Contact dermatitis is a red or blistering rash resulting from contact between the skin and an allergen. Common allergens include poison ivy, poison oak, poison sumac, nickel, cobalt (in jewelry, zippers and metal snaps), neomycin (in antibiotic skin ointment) and latex.

Contact dermatitis can cause skin to be particularly itchy, red and swollen. It may cause fluid-filled blisters or bumps and may weep clear fluid. This condition takes a day or two after exposure to develop. It is not the result of an instant immune system response, as occurs with hives. After an exposure, immune cells called “T-cells” take a day or two to collect at the area of exposed skin and produce chemicals that trigger the rash.

If you develop a contact dermatitis rash, think of the possible triggers you may have been exposed to in the past two to three days, not just on the day that the rash appeared. Once you have identified an allergy that causes contact dermatitis on your skin, your best strategy is to limit symptoms is to avoid a repeat exposure.

Treatment for contact dermatitis can require a topical corticosteroid ointment or (if a large area of skin is affected) oral prednisone. Other anti-inflammatory treatments are available, and may be prescribed by your doctor. Antihistamines may be useful for reducing itching, but they have not been proven to cause improvement in a contact dermatitis rash.

True food allergies affect only about 1 percent of adults, 8 percent of preschool children and 2 percent of older children and newborns. Food intolerances are different from food allergies, and food intolerances are much more common.

Food intolerance usually produces symptoms in the intestinal tract, such as nausea, diarrhea or abdominal pain. Examples of common food intolerances are lactose intolerance or sensitivity to caffeine. Because food intolerance does not involve the immune system, it is not potentially life-threatening like a food allergy. Food allergies can trigger anaphylaxis.

Gastroenteritis, or food poisoning, is another type of food reaction that is sometimes mistaken for food allergy. This one-time reaction is caused by an organism or toxin in food that has been contaminated, undercooked or left at room temperature too long. It doesn't mean that the same food will give you a reaction the next time you eat it. The most common triggers of true food allergies include cow's milk (in infants), eggs, wheat, peanuts, soy, tree nuts, fish and shellfish. Symptoms of food allergy usually begin within minutes to an hour of eating the food allergen:

More severe symptoms can include shortness of breath or wheezing, dizziness, fainting or life-threatening anaphylaxis.

Children can outgrow a food allergy, such as the common allergy to cow's milk. Allergies to peanuts, tree nuts, fish and shellfish, on the other hand, usually persist into adulthood. All children with allergies, particularly teen-agers who spend more time unsupervised by adults, should be educated about what they are allergic to and what to do when they are having symptoms. School personnel, babysitters, child-care providers and the parents of playmates should also be educated this way.

Food allergies of any type at any age require careful attention to what you eat. Learning how to avoid your food allergens is as important as knowing how to treat an allergic reaction with medications. Read food labels carefully. Searching for the word "egg" or "milk" is not adequate. Learn to recognize the different names for your allergen. Sometimes ingredient lists are not complete; learn about product alerts to help avoid allergic reactions.

You may need to avoid eating out at specific restaurants or ordering certain dishes. When you order foods, you need to specify exactly what ingredients you can't eat. For example, a restaurant chef may forget that butter is a dairy product and may use that to cook your food despite your request for no milk products. People with peanut allergies need to avoid most Thai food, because peanuts are used frequently in Thai dishes. You will learn to be incredibly specific when eating out.

Any reaction to a medication is important, whether it is an allergic reaction or not. But health care providers are particularly concerned about allergic reactions to medications because they can lead to a serious, life-threatening reaction called anaphylaxis on subsequent exposures. Certain types of medications are more commonly associated with anaphylaxis than others. These include penicillin, cephalosporins or sulfa drugs. A medication “side effect” that does not involve a rash or throat swelling is different than an allergy. If you experience a medication side effect, you can have the same side effect if you take that medicine or a related medication in the future. However, side effects that do not include allergic symptoms do not make you vulnerable to anaphylaxis.

When you tell your health care provider about any reaction to a medication, he or she will ask you to be specific about your symptoms. Rash, itchy throat, tight throat, trouble breathing or swallowing and anaphylaxis all suggest a true medication allergy. Your description will help your health-care provider determine whether or not you should ever take that medication again.

Some people are allergic to the venom injected into the skin during an insect bite or sting. The more severe and potentially life-threatening insect allergies include reactions to bee, hornet, and wasp stings. Reactions to other insects (such as mosquitoes) tend to be milder.

With or without an allergy to insect venom, anyone can notice a spreading rash at the site soon after being stung or bitten, as well as redness and swelling. The swelling can be very dramatic, particularly if it is on the hands or face. A large local reaction is not itself a sign of an allergic reaction. However, it can mean that you have had a large exposure to insect venom, so you may be more likely to develop allergy newly after you have a large reaction. If you have a large reaction, you and your doctor may decide to do skin tests once your rash has cleared, to see if you have evidence of allergy to insect venom. If you have a true allergy, hives and itching may develop throughout the body. Emergency medical attention is necessary if symptoms of anaphylaxis develop, such as throat tightening or itching, dizziness and trouble breathing; call 911 or your local emergency number immediately.

Deaths from insect stings are uncommon but do happen. It is essential for people with insect allergies to prepare for a dangerous reaction. Ask your health-care provider if you should have an EpiPen, which you can use to administer epinephrine yourself, even before emergency medical help arrives.

Persistent redness or swelling around an insect sting or bite may be a sign of infection and should be evaluated by a health-care provider. Infections tend to be more painful than itchy and may be accompanied by pus around the sting or by fever. Red streaks in the skin near the sting should alert you to the possibility of a serious infection that is spreading quickly; you should seek medical attention right away.

Environmental allergy refers to any allergy to pollen, dust mite deposits, mold, proteins, cockroach exposure, or animal dander. Allergic rhinitis, asthma and atopic dermatitis are common types of reactions to these allergens. The fact that they are called “environmental” allergies highlights that it is important for you to control your surroundings where you live and work, in order to minimize your allergen exposures.

Once you learn that you are allergic to something in the environment, your health-care provider can teach you how to avoid or minimize contact. In addition, you can learn how to eliminate the things in your home or work environment that harbor these allergens.