This article is from the Children Allergies and Asthma FAQ, by Eileen Kupstas Soo firstname.lastname@example.org with numerous contributions by others.
Allergies can show themselves through various skin reactions. The
main reactions are rashes (small bumps or larger red patches), hives
(also called urticaria; itchy, red raised patches on the skin),
or eczema (also called atopic dermatitis; an itchy, weeping rash).
These symptoms can have various causes. Most often it is allergies, but
some people get hives from heat, cold or sun exposure.
Contact allergies/dermatitis is defined as a skin rash caused by
direct contact with a substance to which the skin is
sensitive. Symptoms include a red rash, swelling, and itching. In more
severe cases, blisters can form. Many substances can cause allergic
contact dermatitis: poison ivy and other plants (such as tomato
plants), wool, perfumes and dyes (in soaps, detergents, lotions,
etc.), metals (in jewelry, hair clips, etc.), locally applied
medicinal ointments such as antibiotic creams, and latex (often used
in latex gloves). These can occur at any age and can appear at any
time. It can take years for a sensitivity to a particular substance to
develop, so "I've used this for years" isn't a reason to exclude
anything from the possible allergen list. Symptoms may appear as soon
as 7 to 10 days from first contact. Once a sensitivity develops,
however, the reaction can occur in 24-48 hours.
Treatment of contact dermatitis generally consists of avoiding the
allergen. To determine whether or not something is the cause, patch
tests (a small amount of the substance applied to the skin, then
covered and left for 24 hours) can often show whether or not that
substance causes the reaction.
Other allergies can cause rashes, too. Some find that citrus fruits cause
small raised bumps when ingested.
Eczema is often caused by a food allergy, though there may be other
causes. Cow's milk is a particularly common allergen for those with
eczema. Avoiding allergens provides long-term relief, while short-term
relief can be had by using moisturizers on the skin and taking
antihistamines. Some find that using all cotton clothing and bedding
makes a difference.
Hives can be caused by a number of factors, not just allergies. Hives
occur suddenly and may end suddenly, though there are chronic cases
where hives are present for a month or more. Other causes of hives are sun
exposure, heat and cold. Again, avoidance is the primary treatment.