Allergic reaction


An allergic reaction is a reaction of the body to a certain non-infectious reagent – an allergen – which it recognizes as foreign to the body and to which it reacts by releasing certain substances. These include substances that activate the immune system as well as inflammatory mediators that cause various symptoms in the affected person. Allergic reactions can be divided into different subtypes and can take a mild to life-threatening course.


An allergic reaction is usually caused by a general predisposition to react hypersensitively to foreign, actually harmless substances. On first contact with the allergen, the body then forms antibodies against this substance, as it classifies it as foreign and dangerous. This phase is also called sensitization and does not itself trigger any symptoms.

However, if there is a subsequent renewed contact with the allergen, an allergic reaction can develop with itching, wheals, reddening of the skin, a drop in blood pressure and life-threatening cardiovascular weakness. The extent of the allergic reaction depends on the extent to which the inflammatory substance is released, or the extent of the physical reaction at the cellular level. This fundamental tendency to develop allergies is also known in medicine as the atopic tendency.

Other factors that could contribute to the development of an allergy are controversially discussed. However, the general predisposition seems to play an essential role. Antibiotics are drugs against bacterial infections, which particularly often cause allergic reactions.

Penicillin allergy is particularly frequent. After taking the antibiotic, affected persons usually develop a pronounced skin rash with agonizing itching and wheals. In the case of immediate reactions, these symptoms occur promptly after taking the drug, usually within the first hour.

However, there are also late reactions in which the first symptoms only appear after a few hours or even days. The leading symptom is usually a so-called drug exanthema – a pronounced skin rash that often does not heal for several days. This too can take on various degrees of severity, including life-threatening forms.

Once an allergic reaction to an antibiotic has been diagnosed, it should be avoided if possible. This is especially true in the case of severe allergic reactions. Persons affected should inform any doctor treating them of their allergy so that he or she does not prescribe the wrong antibiotic.

Associated symptoms

The accompanying symptoms of an allergic reaction are manifold. Particularly typical are itching and redness as well as wheals on the skin, watery and itchy, reddened eyes, a runny nose and frequent sneezing. Coughing can also occur.

The mucous membranes swell in the event of more severe allergic reactions. This can cause breathing difficulties and even suffocation. Very severe allergic reactions are accompanied by fluid retention in all tissues and cause circulatory reactions with a drop in blood pressure up to circulatory failure.

These symptoms are life-threatening and require immediate medical treatment. Food allergies can also cause diarrhoea and abdominal pain. General symptoms of an allergic reaction are also general fatigue and reduced performance.

Allergic reactions are often accompanied by wheals on the skin. So-called mast cells, which are located in the upper layers of the skin and react to substances with which the human body comes into contact, play an important role in the development of these wheals. If these cells identify an allergen as foreign to the body, they release special substances – including histamine and leukotrienes.

These substances cause fluid to leak from nearby blood vessels and accumulate under the skin. This tissue fluid becomes visible from the outside as wheals. As a rule, wheals disappear again by themselves after a short time without any treatment being necessary.

The itching that typically occurs in allergic reactions is also related to the activation of mast cells in the skin. Mast cells release histamine and other messenger substances that irritate the nerve fibres in the tissue and the surrounding cells. Itching is the result.

In addition, the released messenger substances in turn stimulate further mast cells to release their histamine into the environment. This cascade leads to an exponential release of signal substances into the cell’s environment and causes the unpleasant symptoms. The itching continues until all mast cells in the affected area have released their messenger substances and these have been broken down again in the tissue.