Allergies are the immune system’s adverse reactions to a foreign substance. Exposure to what is normally a harmless compound, such as pollen, causes the immune system to react as if it were deleterious. Substances that cause allergies are called allergens.

Many substances can cause allergic reactions, including:

• Tree- and grass pollens
• Animal hair, tissue or feces (e.g., cat or dog)
• Insects (e.g., mites) and insect venom (e.g., bee or wasp)
• Foods of plant (e.g. vegetables) or animal (e.g., cow’s milk) origin
• Chemical substances (e.g., drugs 

Over the last 2 decades, the prevalence of allergic diseases has increased dramatically in the industrialized world. In Europe, one out of three is estimated to suffer from at least one of the many symptoms of allergy. Some scientists believe that this development is based on a dramatic life style change provoked by an increased daily hygiene or by the eradication of certain pathogens from our environment. Others state that the disease is triggered by increasing concentrations of pollutants in our environment. Apart from a likely environmental contribution, it has been shown in scientific studies that the development of allergy correlates to the presence of certain risk genes and this heritable component is particularly evident in families where both parents are affected, leading to an at least 50% chance for the offspring to develop the disease. Nevertheless, any genetic component only confers an increased risk whereas the actual development of the disease depends on a number of parameters, including the repeated exposure to elevated concentrations of allergens, amongst others. Food allergies are particularly common among children below 3 years of age, whereas the corresponding symptoms usually vanish with adolescence. In contrast to this, adults are especially prone to becoming sensitized to environmental allergens such as cat hair, mites or pollen.

Looking at it from the view of an aggrieved party allergies are distinguished especially in following categories by the way of ingestion

The following is a typical classification of allergic reactions:

• Inhalant allergies (e.g. to pollen or spores)
• Food allergies (e.g., to fruits, vegetables or dairy products)
• Drug allergies (e.g. to antibiotics)
• Insect venom allergies (e.g. to bee or wasp)
• Contact allergies (e.g. to metals)

How do allergies develop?

After the primary contact with a particular biological source (e.g. pollen), a patient’s immune system develops so-called IgE antibodies against one or several allergen molecules. This is called sensitization. Upon the second encounter with the same allergen, sometimes years later, the immune system responds with a molecular chain reaction leading to the cross-reaction of IgE molecules bound to the surface of so-called mast cells. As a result, a vast number of mediators of inflammation are released (e.g. histamines) triggering the well-known symptoms of allergy.

What are the typical symptoms of allergy and how do they present ?

Allergy can affect different organs and tissues in the human body, leading to different symptoms.
The main symptoms of allergic diseases are:

• (Chronic) rhinitis ⁄  hay fever
  Hay fever, otherwise known as seasonal allergic rhinitis, is an allergic reaction to airborne substances such as pollen that get into the upper respiratory passages - the nose, sinus, throat – and also the eyes. The name hay fever is misleading because symptoms don't just occur in autumn when hay is gathered and never include fever. „Allergic rhinitis“ is the correct term used to describe this type of allergy. Rhinitis means „irritation of the nose“ and is a derivative of „Rhino“, meaning nose. Allergic rhinitis which occurs during a specific season is called ”seasonal allergic rhinitis“. Symptoms of allergic rhinitis, or hay fever, frequently include nasal congestion, a clear runny nose, sneezing, nose and eye itching, and tearing eyes. Hay fever is the most common of all the allergic diseases – about 15 per cent of the population in industrialized countries suffer from this condition. Symptoms usually appear in childhood first and then lessen by the age of 30 or 40. Perennial allergic rhinitis is a similar allergy that occurs all year round and is caused by things such as house dust mites and pets. However the predominant allergen changes from time to time.
• Conjunctivitis
  Inflammation of the conjunctiva (conjunctivitis) is often caused by allergens and it frequently presents in conjunction with rhinitis. Allergic conjunctivitis is common in people who have other signs of allergic disease such as hay fever, asthma and eczema. Conjunctivitis is often caused by antigens like pollen, dust mites or cosmetics. Symptoms include burning, discharge, dryness, itching, light sensitivity, eye pain or discomfort, stickiness, tearing and chemosis. Rubbing of the eye can possibly worsen symptoms.
• Shortness of breath, asthma
  In some allergic patients, asthma symptoms can be triggered by inhaling particular environmental triggers (e.g. allergens or other environmental pollutants) leading to an inflammatory reaction in the airways of the lung. Typical asthma-eliciting allergens include pet dander, dust mites, cockroach allergens, molds, or pollens. Asthma symptoms can also be triggered by respiratory infections, exercise, cold air, tobacco smoke and other pollutants, stress, food, or drug allergies. When an asthma attack occurs, the muscles surrounding the airways become tight and the lining of the air passages swell. This reduces the amount of air that can pass by, and can lead to wheezing sounds. Most people with asthma have wheezing attacks separated by symptom-free periods. Some patients have long-term shortness of breath with episodes of increased shortness of breath. Still, in others, a cough may be the main symptom. Asthma attacks can last minutes to days and can become dangerous if the airflow becomes severely restricted. Smoking or physical activities.
• Allergic reactions of the skin
  The skin is our most important barrier when protecting the body from getting in contact with pathogens or pollutants and therefore it is also a key contact point for allergens. Allergic contact dermatitis is an itchy skin condition caused by an allergic reaction to material in contact with the skin. It arises some hours after contact with the responsible material, and settles down over some days providing the skin is no longer in contact with it. Contact dermatitis should be distinguished from contact urticaria, in which a rash appears within minutes of exposure and fades away within minutes to hours. The allergic reaction to latex is the best known example of allergic contact urticaria. Allergic contact dermatitis is also distinct from irritant contact dermatitis, in which a similar skin condition is caused by excessive contact with irritants. Irritants include water, soaps, detergents, solvents, acids, alkalis, and friction. Irritant contact dermatitis may affect anyone, providing they have had enough exposure to the irritant, but those with atopic dermatitis are particularly sensitive. Most cases of hand dermatitis are due to contact with irritants.
• Circulation problems, fatique
  The response issued by the human body’s immune system in response to allergen contact (e.g. a local inflammation) can have a profound effect on the circulation or lead to a general fatigue. Frequently, these symptoms are caused by drugs such as anti–histamines.
• Digestive symptoms
  An allergy can also affect the the digestive system may cause in which case it may present with symptoms of stomach pain, nausea, vomiting or diarrhea.
• Anaphylaxis
  Anaphylaxis is a severe allergic reaction that affects the entire immune system simultaneously with possibly life– threatening results. Full-blown anaphylaxis includes urticaria (hives) and/or angioedema (tissue swelling) with hypotension (low blood pressure) and bronchospasm (asthma). Anaphylactoid reactions are clinically similar to those experienced in anaphylaxis. The only difference is that anaphylactoid reactions are non-IgE mediated and may be less severe. Severity of reactions from anaphylaxis can vary from mild symptoms to sudden death. In any case, medical attention should be sought immediately and appropriate treatment provided.