Allergic contact dermatitis is a forms
of contact dermatitis that is the
manifestation of an allergic response
caused by contact with a substance; the
other type being irritant contact
dermatitis.
Although less common than ICD, ACD is
accepted to be the most prevalent form
of immunotoxicity found in humans. By
its allergic nature, this form of
contact dermatitis is a hypersensitive
reaction that is atypical within the
population. The mechanisms by which
these reactions occur are complex, with
many levels of fine control. Their
immunology centres on the interaction of
immunoregulatory cytokines and discrete
subpopulations of T lymphocytes.
Signs and symptoms 
The symptoms of allergic contact
dermatitis are very similar to the ones
caused by irritant contact dermatitis,
which makes the first even harder to
diagnose. The first sign of allergic
contact dermatitis is the presence of
the rash or skin lesion at the site of
exposure. Depending on the type of
allergen causing it, the rash can ooze,
drain or crust and it can become raw,
scaled or thickened. Also, it is
possible that the skin lesion does not
take the form of a rash but it may
include papules, blisters, vesicles or
even a simple red area. The main
difference between the rash caused by
allergic contact dermatitis and the one
caused by irritant contact dermatitis is
that the first one tends to be confined
to the area where the trigger touched
the skin, whereas in the second case,
the rash is more likely to be more
widespread on the skin. Another
characteristic of the allergic contact
dermatitis rash is that it usually
appears after a day or two after
exposure to the allergen, unlike
irritant contact dermatitis that appears
immediately after the contact with the
trigger.
Other symptoms may include itching, skin
redness or inflammation, localized
swelling and the area may become more
tender or warmer. If left untreated, the
skin may darken and become leathery and
cracked. Pain can also be present.
The symptoms of allergic contact may
persist for as long as one month before
resolving completely. Once an individual
has developed a skin reaction to a
certain substance it is most likely that
they will have it for the rest of their
life, and the symptoms will reappear
when in contact with the allergen.
Cause 
Common allergens implicated include the
following:
Nickel – metal frequently encountered in
jewelry and clasps or buttons on
clothing
Gold – precious metal often found in
jewelry and dental materials
Balsam of Peru – used in food and drink
for flavoring, in perfumes and
toiletries for fragrance, and in
medicine and pharmaceutical items for
healing properties; derived from tree
resin. It may also be a component of
artificial vanilla and/or cinnamon
flavorings.
Chromium – used in the tanning of
leather. Also a component of uncured
cement/mortar, facial cosmetics and some
bar soaps.
Oily coating from plants of
Toxicodendron genus – poison ivy, poison
oak, and poison sumac.
Sap from certain species of mangrove and
agave
Thiomersal – mercury compound used in
local antiseptics and in vaccines
Neomycin – topical antibiotic common in
first aid creams and ointments,
cosmetics, deodorant, soap, and pet
food. Found by itself, or in Neosporin
or Triple Antibiotic
Fragrance mix – group of the eight most
common fragrance allergens found in
foods, cosmetic products, insecticides,
antiseptics, soaps, perfumes, and dental
products 
Formaldehyde – preservative with
multiple uses, e.g., in paper products,
paints, medications, household cleaners,
cosmetic products, and fabric finishes.
Often released into products by the use
of formaldehyde releasers such as
imidazolidinyl urea, diazolidinyl urea,
Quaternium-15, DMDM Hydantoin, and
2-bromo-2-nitropropane-1,3-diol.
Cobalt chloride – metal found in medical
products; hair dye; antiperspirant;
metal-plated objects such as snaps,
buttons or tools; and in cobalt blue
pigment
Bacitracin – topical antibiotic found by
itself, or as Polysporin or Triple
Antibiotic
Quaternium-15 – preservative in cosmetic
products and in industrial products.
Colophony – rosin, sap or sawdust
typically from spruce or fir trees
Topical steroid – see steroid allergy
Photographic developers, especially
those containing metol
Topical anesthetics – such as pramoxine
or diphenhydramine, after prolonged use
othiazolinoneMethylchloroisothiazolinone
– preservative used in many personal
care, household, and commercial
products.
Soluble salts of platinum – see
platinosis
Mechanism 
ACD arises as a result of two essential
stages: an induction phase, which primes
and sensitizes the immune system for an
allergic response, and an elicitation
phase, in which this response is
triggered. As it involves a
cell-mediated allergic response, ACD is
termed a Type IV delayed
hypersensitivity reaction, making it an
exception in the usage of the
designation "allergic," which otherwise
usually refers to type I
hypersensitivity reactions.
Contact allergens are essentially
soluble haptens and, as such, have the
physico-chemical properties that allow
them to cross the stratum corneum of the
skin. They can only cause their response
as part of a complete antigen, involving
their association with epidermal
proteins forming hapten-protein
conjugates. This, in turn, requires them
to be protein-reactive.
The conjugate formed is then recognized
as a foreign body by the Langerhans
cells), which then internalize the
protein; transport it via the lymphatic
system to the regional lymph nodes; and
present the antigen to T-lymphocytes.
This process is controlled by cytokines
and chemokines – with tumor necrosis
factor alpha and certain members of the
interleukin family – and their action
serves either to promote or to inhibit
the mobilization and migration of these
LCs. As the LCs are transported to the
lymph nodes, they become differentiated
and transform into DCs, which are
immunostimulatory in nature.
Once within the lymph glands, the
differentiated DCs present the
allergenic epitope associated with the
allergen to T lymphocytes. These T cells
then divide and differentiate, clonally
multiplying so that if the allergen is
experienced again by the individual,
these T cells will respond more quickly
and more aggressively.
White et al. have suggested that there
appears to be a threshold to the
mechanisms of allergic sensitisation by
ACD-associated allergens. This is
thought to be linked to the level at
which the toxin induces the
up-regulation of the required mandatory
cytokines and chemokines. It has also
been proposed that the vehicle in which
the allergen reaches the skin could take
some responsibility in the sensitisation
of the epidermis by both assisting the
percutaneous penetration and causing
some form of trauma and mobilization of
cytokines itself.
= Memory Response =
Once an individual is sensitized to an
allergen, future contacts with the
allergen can trigger a reaction,
commonly known as a memory response, in
the original site of sensitization. So
for example if a person has an allergic
contact dermatitis on the eyelids, say
from use of makeup, touching the contact
allergen with the fingers can trigger an
allergic reaction on the eyelids.
This is due to local skin memory
T-cells, which remain in the original
sensitization site. In a similar
fashion, cytotoxic T lymphocytes patrol
an area of skin and play an important
role in controlling both the
reactivation of viruses and in limiting
its replication when reactivated. Memory
response, or "Retest Reactivity",
usually takes 2 to 3 days after coming
in contact with the allergen, and can
persist for 2 to 4 weeks.
Diagnosis 
Diagnosing allergic contact dermatitis
is primarily based on physical exam and
medical history. In some cases doctors
can establish an accurate diagnosis
based on the symptoms that the patient
experiences and on the rash's
appearance. In the case of a single
episode of allergic contact dermatitis,
this is all that is necessary. Chronic
and/or intermittent rashes which are not
readily explained by history and
physical exam often will benefit from
further testing.
A patch test is a commonly used
examination to determine the exact cause
of an allergic contact dermatitis.
According to the American Academy of
Allergy, Asthma, and Immunology, "patch
testing is the gold standard for contact
allergen identification".
The patch test consists of applying
small quantities of potential allergens
to small patches and which are then
placed on the skin. After two days, they
are removed and if a skin reaction
occurred to one of the substances
applied, a raised bump will be
noticeable underneath the patch. The
tests are again read at 72 or 96 hours
after application.
Patch testing is used for patients who
have chronic, recurring contact
dermatitis. Other tests that may be used
to diagnose contact dermatitis and rule
out other potential causes of the
symptoms include a skin biopsy and
culture of the skin lesion.
Treatment 
Persons who develop the rash and the
other symptoms from a certain trigger
are most likely to have it for the rest
of their lives and detecting and
avoiding the allergen is mandatory in
treating the condition and resolving its
symptoms.
The first step in treating the condition
is applying a damp cloth shortly after
the skin problem first shows to make
sure that all of the irritant has been
removed from the area. In some cases,
the best treatment is to do nothing to
the area.
In mild to moderate cases, patients may
use skin creams containing
corticosteroids to reduce the
inflammation. These creams should be
used carefully and according to the
instructions they come with because when
overused over longer periods of time
they can cause serious skin conditions.
Also, calamine lotion and cool oatmeal
baths may relieve itching. Over the
counter diphenhydramine by mouth is
helpful for night time itching.
Usually, severe cases are treated with
systemic corticosteroids which may be
tapered gradually, with various dosing
schedules ranging from a total of 12 –
20 days to prevent the recurrence of the
rash as well as a topical
corticosteroid. Tacrolimus ointment or
pimecrolimus cream can also be used
additionally to the corticosteroid
creams or instead of these. Oral
antihistamines such as diphenhydramine
or hydroxyzine may also be used in more
severe cases to relieve the intense
itching. Topical antihistamines are not
advised as there might be a second skin
reaction from the lotion itself.
The other symptoms caused by allergic
contact dermatitis are generally eased
with wet dressings and drying lotions to
stop the itching. In most cases however,
medication or actual treatment is not
required as long as the trigger has been
identified and avoided. The discomfort
caused by the symptoms may be relieved
by wearing smooth-textured clothing to
avoid more skin irritation or by
avoiding soaps with perfumes and dyes.
Commonly, the symptoms may resolve
without treatment in 2 to 4 weeks but
specific medication may hasten the
healing as long as the trigger is
avoided. Also, the condition might
become chronic if the allergen is not
detected and therefore it is not
avoided.
References 
^ Basic Pathology - Robbins et al - 9th
edition
