Why do allergic reactions primarily affect the neck?

Redness, itching, swelling, and even small spots : these are the main symptoms of a skin allergy. As a reminder, we talk about allergy when the immune system reacts excessively to a normally harmless external substance, referred to as an allergen. When the body perceives this substance as a potential threat, it releases histamine, an inflammatory mediator that, by attaching to its specific receptors, triggers the various biological mechanisms leading to the symptoms of the allergy.

During an allergic response, it is primarily the immunoglobulin E, antibodies, that are called upon as these are the immune system proteins that recognise the antigens carried by allergens. After the antigens are recognised, a mast cell degranulation occurs, leading to the production of histamine but also leukotrienes and pro-inflammatory cytokines, compounds that increase vascular permeability and stimulate nerve endings. These various processes are the cause of the characteristic symptoms of skin allergies, which can affect one or more areas of the body, particularly the neck.

It is somewhat challenging to explain why the neck is so susceptible to skin allergies. However, several hypotheses can be proposed:

• The skin on the neck is relatively thin and devoid of sebaceous glands.

  Similar to the skin on the face, the skin on the neck is quite thin, making it particularly vulnerable to irritants and allergens from the environment. Moreover, the neck is characterised by a lower density of sebaceous glands, the organs responsible for the synthesis of sebum. This sebum is a component of the natural hydrolipidic film present on the skin's surface to protect it from dehydration as well as the penetration of pathogens and allergens. The sparse number of sebaceous glands on the neck weakens its hydrolipidic film, making the skin more permeable in this area. This facilitates the penetration of allergenic substances, hence allergic reactions are more pronounced here.

• The neck is quite exposed to external elements.

  It is also plausible to suggest that the tendency of the neck to be the site of allergic reactions is due to its high exposure to external elements, such as dust, pollen or various pollutants. In interaction with the fragrances frequently applied to this area of the body, it is not uncommon for these external factors to trigger skin allergies of the type contact dermatitis.

  Furthermore, the neck is often in contact with metal jewellery, such as nickel, a well-known allergen. According to a recent study conducted with 1015 people prone to allergies, 23.8% of this population have a nickel allergy. Red patches, sometimes topped with blisters, as well as sensations of itching and irritation are characteristic of a nickel allergy. Thus, wearing nickel necklaces can potentially trigger a localised allergic reaction on the neck.

• The skin on the neck is sensitive to shaving irritations.

  Men who shave their beards know this: small cuts can easily occur. Often without consequences, these irritations can damage the skin barrier and increase its permeability to allergens, which heightens the risk of allergic reactions on the neck. Indeed, once these substances have penetrated the epidermis, they can trigger a local immune response.

The neck is an area frequently subjected to friction, particularly during the donning or removal of clothing items such as jumpers, scarves, neckerchiefs. These repeated rubbings can exacerbate allergic reactions.

• SPENCER A. & al. Analysis of Facial Skin Thickness: Defining the Relative Thickness Index. Plastic and Reconstructive Surgery (2005).

• PUCCI S. & al. Allergy and the skin. Clinical and Experimental Immunology (2008).

• SIMON D. & al. Update on skin allergy. European Journal of Allergy and Clinical Immunology (2014).

• TOSTI A. & al. Common Allergens Identified Based on Patch Test Results in Patients with Suspected Contact Dermatitis of the Scalp. Skin Appendage Disorders (2017)