Causes

The cause of a skin rash due to allergy is, as with other allergic reactions, a sensitization of the body to substances that are actually harmless. These are called allergens. This sensitization takes place in two phases, whereby the first phase remains without symptoms.

During this first contact, the body’s own immune cells are formed, which trigger an immune reaction when the respective substance is recognized again. At the second contact, this immune reaction then causes e.g. contact eczema. The body, in this case the defence cells present in the skin, treat the substance as a threat.

The T-cells are mainly responsible for this erroneous reaction (see: white blood cells). Once they have been sensitized to the apparently harmful allergen, they trigger an inflammatory reaction, such as histamine, with each subsequent contact by releasing messenger substances. This process must be distinguished from so-called toxic or irritant contact eczema, which is caused by overloading the skin with actually harmful substances.

In the case of allergic contact eczema, the reaction is triggered by actually harmless substances. Very often these are metals such as nickel or chromate, but also fragrances and preservatives, latex and rubber products or various cosmetics. A food allergy can also be a trigger, and in this case the rash can often be rather diffuse.If the rash occurs mainly after consumption of milk and dairy products, there may be a milk intolerance. Another trigger for allergy-like symptoms can be so-called pseudoallergies, such as sweat allergy.

Skin rash causes
Cross Allergy
Skin rash from tomatoes
Symptoms of nickel allergy

Diagnosis

The diagnosis is mainly based on the skin findings in combination with the medical history. However, it can be difficult to establish a connection with the triggering substance or point in time, especially in the case of prolonged occupational sensitization. In addition, even with the help of a skin biopsy and subsequent analysis, it is not possible to tell whether the trigger was allergic or toxic/irritant because of the skin change itself.

However, indications of this can be found very well with common allergy tests (see: Main article Allergy Diagnostics). This includes the epicutaneous test (or patch test). In this test, potentially allergenic substances are applied to the back using special patches. A possible skin reaction can then be read off, usually after 48 or 72 hours. When used correctly, a large number of allergens can be detected or excluded.