How to distinguish allergy spots from other skin eruptions?

Allergy spots are the result of the immune system's intolerance towards a substance it cannot tolerate. When this substance, commonly referred to as an allergen, comes into contact with the skin or is ingested, as can be the case with food allergies, it is recognised by the immune system's immunoglobulins, which trigger the release of histamine and other inflammation mediators, such as interleukins IL-4. The build-up of these molecules in the tissues causes a dilation of the blood vessels and an accumulation of fluid in the surrounding tissues, leading to swelling of the affected area and the appearance of spots on the skin's surface.

Spots of an allergic origin are often red, raised, and associated with itching. The progression of allergy spots is generally quite rapid: these lesions can develop, change shape, or disappear within a few hours, as the allergic reaction evolves. This phenomenon is particularly characteristic of urticaria, but can also be seen in other types of skin allergies, such as contact dermatitis.

Allergic spots can resemble other types of skin rashes, such as those seen in acne, rosacea, or certain viral infections, but each type of spot has specific characteristics that allow them to be differentiated.

• Evaluating the speed of onset of spots.

  Allergic spots often form quickly after contact with the responsible allergen. For instance, those appearing as part of urticaria are generally visible a few minutes after exposure to the allergen. Conversely, spots related to dermatoses, such as acne or rosacea, tend to develop over several days. As for spots resulting from viral infections, like chickenpox, they form following specific phases. Chickenpox begins with small red spots that quickly transform into fluid-filled vesicles, unlike allergic spots which generally remain as papules or raised, red plaques.

• Detailing the appearance of spots.

  The appearance of spots can be a good indicator to determine whether they have an allergic cause or not. Indeed, allergy spots often take the form of raised red papules, surrounded by a slight swelling. Small vesicles, characteristic of contact eczema, can also form. In the case of urticaria, it is quite common to observe raised patches. In all cases, these spots generally form clustered eruptions.

  Allergic spots can resemble chickenpox spots, initially taking the form of small red papules. However, these later transform into vesicles and form crusts, unlike allergy spots. As for acne spots, they are quite characteristic and are more akin to open or closed comedones, papules, pustules, or, in more severe cases, nodules and cysts. The spots associated with papulopustular rosacea correspond to papules and pustules. Finally, spots due to folliculitis are small pustules surrounded by redness.

• Observe the location of the spots.

  The location where spots appear is another clue that can help determine their allergic nature. Indeed, spots resulting from a contact allergy are located in the areas of contact with the allergen, such as the neck when wearing nickel jewellery, a common allergen. The hands are also often the site of contact allergies as they are frequently exposed to external elements. Spots following a food allergy generally spread across the entire body.

  Conversely, acne spots tend to form on the face, back and chest, where the sebaceous glands are most numerous. Rosacea spots, primarily located on the face, are often situated around the cheeks, nose, forehead and chin, and are accompanied by widespread redness. Those due to viral infections, such as chickenpox, first appear on the trunk and face, before spreading to the rest of the body. Finally, spots resulting from folliculitis form on areas with hair, such as the beard or scalp.

• Questioning about pruritus.

  Allergy spots are often accompanied by intense itching, caused by the release of histamine and other inflammatory mediators by immune cells. This distinguishes them from skin rashes due to acne or rosacea, which are generally painless. On the other hand, spots caused by viral infections such as chickenpox tend to itch, especially in the early stages.

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