What are Allergies? Common Symptoms of Allergic Reactions

Definition of an allergy

An allergy is an immune system disorder that occurs when the body mistakenly identifies normally harmless substances (allergens) as dangerous threats. In essence, this is an exaggerated immune response to harmless triggers such as pollen, food, medications, fine dust, or animal dander.

According to the World Health Organization (WHO), allergies are classified as a group of chronic diseases, which include conditions such as allergic rhinitis, allergic asthma, atopic dermatitis, and food allergies. These disorders often cause a wide range of uncomfortable symptoms, including skin itching, sneezing, runny nose, abdominal pain, or diarrhea.

While most allergic reactions are mild and mainly cause discomfort, in severe cases, the response can progress to anaphylaxis – a life-threatening medical emergency.

What causes allergies to develop?

The causes of allergies are generally related to genetic predisposition and environmental factors.

• Genetic factors: A family history of atopic disease (such as asthma, allergic rhinitis, or atopic dermatitis) increases the likelihood of developing allergies.
• Environmental factors: Immune sensitization may be promoted by air pollution, dietary pattern shifts, and reduced early-life microbial exposure (e.g., limited contact with common bacteria and environmental microbiota).

Immunopathogenesis

Allergy is not an “instant” event but a structured immune process with two principal phases.

• Sensitization phase: On first exposure, the immune system “records” the allergen, often via Th2-skewed responses, inducing production of allergen-specific IgE.
• Raction phase: Upon re-exposure to the remembered allergen, IgE bound to mast cells and basophils is cross-linked, triggering release of mediators. This leads to sneezing, rhinorrhea, pruritus, urticaria, dyspnea, etc.

You can think of allergy as a false alarm system that repeatedly blares at commonplace, non-dangerous stimuli.

Common types of allergies in human

Allergies are not a single condition but are classified according to the triggering allergen. Common forms include respiratory allergies, skin allergies, food allergies, drug allergies, and insect venom allergies.

Respiratory allergy

A respiratory allergy occurs when a person inhales allergens such as pollen, house dust mites, mold spores, or animal dander. The immune system reacts abnormally to these particles once they enter the airways, leading to nasal congestion, mucus hypersecretion and breathing difficulty.

Typical clinical symtoms of respiratory allergy include:

• Persistent sneezing.
• Rhinorrhea.
• Nasal congestion.
• Itching of the nose, throat, or eyes.
• Wheezing, shortness of breath, chronic cough.

Food allergy

A food allergy occurs when the immune system misidentifies certain foods, such as cow’s milk, eggs, seafood, or peanuts, as harmful. Ingesting these foods triggers immune cells to release histamine and other inflammatory mediators, resulting in a rapid allergic response involving the gastrointestinal tract, skin and respiratory system.

Typical clinical symptoms of food allergy include:

• Pruritus or angioedema of the lips, tongue, oropharynx.
• Urticaria, morbilliform rash.
• Abdominal pain, diarrhea, nausea, vomiting.
• Nasal congestion or dyspnea.
• Dizziness, hypotension.

Drug allergy

A drug allergy is an abnormal immune response in which the body identifies a medication component (e.g., penicillin, aspirin, anesthetic agents) as harmful. The immune system produces drug-specific antibodies or activates immune cells, leading to a spectrum of reactions that are often mild but can be life-threatening.

Typical clinical symptoms of drug allergy include:

• Erythematous rash, pruritus.
• Angioedema of the lips, eyes, face.
• Low-grade fever, malaise.
• Wheezing, dyspnea.
• Anaphylaxis with hypotension, tachycardia.

Insect venom allergy

This allergy occurs when venom from insects such as bees, ants, or wasps enters the body. In predisposed individuals, the immune system mounts a marked systemic response to the venom, causing local swelling and erythema or, in severe cases, generalized reactions.

Typical clinical symptoms of insect venom allergy include:

• Localized swelling, erythema, burning pain at the sting site.
• Generalized pruritus or urticarial eruptions.
• Dyspnea, wheezing.
• Dizziness, hypotension.
• Anaphylaxis in severe cases.

Skin allergy

Skin allergy commonly arises when skin contacts chemicals, cosmetics, metals (e.g., nickel) or latex. Cutaneous immune surveillance misidentifies these haptens and triggers delayed-type hypersensitivity, typically hours after exposure.

Typical clinical symptoms of skin allergy include:

• Erythema, intense pruritus at the contact site.
• Papules or micro-vesiculation.
• Xerosis, scaling, or fissuring.
• Chronic dermatitis if exposure persists.

Individuals at higher risk of developing allergies

Not everyone has the same susceptibility to allergic diseases. Numerous scientific studies show that allergies tend to occur more frequently in individuals with certain genetic, biological, or environmental risk factors.

• Infants, young children, adolescents.
• Individuals with a family history of atopic diseases.
• Individuals living in polluted environments.
• Active smokers or those exposed to secondhand smoke.
• Individuals with underlying immune or respiratory disorders, including asthma, atopic dermatitis, allergic rhinitis, etc.
• Individuals with a heightened immune reactivity to medications.
• Individuals with diets are high in processed foods.
• Individuals with limited exposure to natural environmental microbes.
• Children with early-life health conditions such as cesarean delivery, premature birth, or lack of exclusive breastfeeding.

Complications of untreated allergies

If allergies are not properly managed, they can lead to a range of serious medical complications.

• Bronchial asthma: Chronic airway inflammation and bronchoconstriction may develop, causing persistent wheezing, coughing, and shortness of breath. Poorly controlled asthma can lead to acute severe asthma attacks, which may become life-threatening.
• Chronic rhinosinusitis: Prolonged nasal obstruction and mucus retention can lead to chronic inflammation of the paranasal sinuses. Patients often present with nasal congestion, facial pain and persistent fatigue.
• Otitis media with effusion: Children are particularly prone to middle ear effusion when mucus from the nasal cavity and sinuses accumulates in the middle ear. If left untreated, it can cause hearing loss, speech delay and impaired learning.
• Nasal polyps: Long-standing mucosal inflammation may lead to the formation of soft, benign nasal polyps that obstruct the airway. Patients often experience nasal blockage, reduced sense of smell and recurrent respiratory infections.
• Impact on physical development: Children with chronic allergies and long-term nasal congestion may have their sleep and eating affected, leading to stunted growth in height and weight. Some children are also at risk of having facial skeletal changes (e.g., long face, dental malocclusion) due to prolonged mouth breathing.
• Anaphylaxis: A rapid-onset, severe systemic allergic reaction that can occur after exposure to medications, foods, or insect venom. It is characterized by hypotension, airway compromise, and can be fatal within minutes if not treated promptly with emergency care.

Diagnosis of allergic conditions

Diagnosing allergy cannot rely on symptoms alone; it requires a combination of detailed clinical history and specialized diagnostic tests. This is essential because many non-allergic conditions, such as food intolerance, mechanical irritation, infections, may present similar manifestations.

Tests help determine the type of allergy, severity of the immune response and the exact allergen involved. In some cases, controlled allergen exposure under medical supervision is necessary to confirm the diagnosis.

Common diagnostic tests include:

• Skin prick test (SPT): Detects immediate-type hypersensitivity to inhalant or food allergens by observing local wheal-and-flare reactions.
• IgE blood test: Measures serum IgE antibodies against specific allergens.
• Total IgE blood test: Evaluates the overall atopic tendency of the body.
• Patch test: Identifies delayed-type contact allergy, especially to detergents, metals, cosmetics, latex, etc.
• Allergen challenge test: Controlled exposure to the suspected allergen in a medical setting, mostly for oral food and drug allergy.

Effective treatment and management for allergies

Allergy treatment goes beyond simply relieving symptoms. It requires a comprehensive approach, including allergen avoidance, pharmacologic therapy, lifestyle modification, and in selected cases, allergen-specific immunotherapy.

• Allergen avoidance: Eliminate known food allergens, control environment exposure, and avoid tiggering products including cosmetcs, detergents or latex-containing items.
• Medication: This is the first-line option for rapid and effective symptom control, including antihistamines (reduce pruritus, sneezing, rhinorrhea), corticosteroids (reduce inflammation in rhinitis, dermatitis, asthma), bronchodilators (for respiratory allergy), and adrenaline (EpiPen) for anaphylaxis.
• Allergen-specific immunotherapy: This treatment is recommended when symptoms persist despite optimal medication or when there is a high risk of severe progressive allergy. Allergen-specific immunotherapy often lasts 3 – 5 years, help reduce symptom severity and prevent progression to chronic asthma or persistent allergic rhinitis.

In addition, allergy management should also include supportive measures such as maintaining a balanced, immune-boosting diet and controlling coexisting conditions. This is important because certain underlying disorders, such as asthma or atopic dermatitis, can trigger recurrent allergy if not properly managed.

Frequently asked questions about allergies

What is an acute allergic reaction?

An acute allergic reaction is a rapid-onset hypersensitivity response that occurs within minutes to a few hours after exposure to an allergen. Patients may experience sudden symptoms such as urticaria, pruritus, rhinorrhea, angioedema of the lips/eyelids/throat, dyspnea, and life-threatening anaphylaxis in severe cases.

How does an acute allergic reaction differ from a typical allergy?

An acute allergic reaction occurs very rapidly within minutes to a few hours after exposure to an allergen and typically presents with sudden, pronounced symptoms such as generalized urticaria, angioedema, shortness of breath, or life-threatening anaphylaxis.

Typical allergies progress slowly, recur over time and tend to persist. Examples include seasonal allergic rhinitis, atopic dermatitis, or allergic asthma, which mainly cause discomforting symptoms but rarely pose an immediate life-threatening risk.

Is it concerning to suddenly develop an allergy?

The sudden onset of allergy can be worrying, but not every case is dangerous. However, if you experience severe symptoms such as shortness of breath, angioedema, or signs of anaphylaxis, you should seek immediate medical attention for prompt evaluation and treatment.

What is an allergy history?

An allergy history is a record of past allergic reactions, including the type of allergen involved (e.g., medications, foods, pollen), the severity of the reaction, and the treatments that were used.

What types of allergic contact dermatitis exist?

Skin allergy presents in various forms, including urticaria, allergic contact dermatitis, atopic dermatitis and angioedema. These conditions can range from mild to severe, depending on an individual’s immune sensitivity and the level or duration of allergen exposure.

What should you do if you have an allergic reaction?

The first step is to identify and avoid the allergen whenever possible. Then, monitor your symptoms closely and seek medical consultation promptly so a physician can prescribe appropriate medication to control the reaction.

Do not underestimate allergy symptoms. Many cases will not resolve on their own and may persist or worsen, significantly affecting your health and quality of life.

In addition, you should consider proactive allergy screening. This is an effective way to identify potential triggers in advance, allowing you to adjust your lifestyle and diet to reduce risk and better manage future allergic reactions.

Currently, DIAG is one of the most trusted providers of allergy testing, offering not only screening for more than 72 allergens quickly and accurately but also personalized dietary guidance to help you maintain optimal health.

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