Allergic Contact Dermatitis 2020-11-27T16:08:46+00:00

Allergic Contact Dermatitis

What is allergic contact dermatitis?

An allergic or hypersensitivity reaction occurs when a person becomes unusually sensitive to a substance (called an allergen) that is typically well tolerated by the wider population. The result can be a red, inflamed and itchy rash on the skin. There are 4 main types of allergic reaction, identified as type I, type II, type III, and type IV. The first three types are known as antibody-mediated immune reactions, whilst type IV is a cell-mediated reaction and forms the basis for allergic contact dermatitis. The difference between these two types of reaction lies in the way the body is attacked, and how it reacts.

What causes it?

Allergic contact dermatitis (ACD) arises as a result of repeated exposure to small doses of an allergen. Common allergens in hair and beauty products include the hair dyes paraphenylenediamine and paratoluenediamine, and the preservative methylisothiazolinone.

The condition results from a malfunction of the body’s autoimmune system. During each exposure, the dendritic cells of the epidermis (the skin) start to adapt and cause T lynphocytes to ‘memorise’ the allergen – a process called sensitisation.

This causes an increase of T-cell production in local lymph nodes, which then circulate around the whole of the body. With subsequent exposure, T-cells at the site where the allergen entered the skin trigger a local inflammatory response.

The symptoms

The symptoms of ACD can vary, depending on the severity of the attack There may be a raw, scaly and inflamed rash on the skin, with more severe cases resulting in the skin being blistered and weeping.

Importantly, the symptoms will appear some 12-72 hours after exposure to the allergen, which is the time the body’s cells take to respond to the allergen. This is why the ACD response is known as a delayed hypersensitivity reaction. This is one way to distinguish ACD from irritant contact dermatitis (ICD), when the response in the form of a rash and inflammation occurs more rapidly after contact with the problem substance and also decays more quickly than ACD.


The effects of the ACD outbreak may be treated with topical steroids to counter the inflammation. But once the body is sensitized to an allergen, and the allergen has been identified, the most effective treatment is to avoid future exposure.

Why see a trichologist?

A trichologist will examine the skin and take a history of the symptoms. In cases of ACD, the trichologist can advise on likely allergens and therefore help put in to place the necessary avoidance measures.

The trichologist will also recommend referral to a dermatologist to identify the allergen or allergens which have caused the ACD.

More facts

ACD should not be confused with irritant contact dermatitis (ICD).

ICD does not engage the immune system and is a more instant and temporary condition that tends to be relieved once the irritant chemical or substance has been removed.

By contrast in ACD, once an individual becomes sensitized to a certain allergen, they will remain allergic to that chemical or substance for life.


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