Allergies

Overview of Allergies

Allergies are exaggerated, misdirected immune responses against normally harmless substances from the environment, called allergens (for example pollen, components of house dust, or certain foods). In allergic individuals, the immune system treats these substances as if they were dangerous pathogens and mounts a specific immune reaction that can damage the body.

Allergies belong to the broader group of hypersensitivity reactions. What makes allergies special is that:

They involve specific immune recognition of an allergen.
The reaction usually appears only after prior sensitization (first contact).
Symptoms are often caused not by the allergen itself, but by the body’s own inflammatory mediators (such as histamine).

In other sections, different types of hypersensitivity, autoimmune diseases, and basic immune mechanisms are discussed. Here, the focus is on allergic disease specifically.

Allergens and Routes of Exposure

Allergens are typically proteins (or substances bound to proteins) that can be recognized by the immune system. Common features:

They are often soluble and small enough to reach immune cells.
They are frequently present in the environment in tiny amounts but repeatedly.

Typical allergen groups and how they enter the body:

Inhalant allergens (via airways)

Pollen from grasses, trees, weeds
House dust mites and their feces
Animal dander (skin flakes, saliva proteins)
Mold spores
→ Can cause allergic rhinitis (hay fever), allergic asthma.

Food allergens (via digestive tract)

Cow’s milk proteins, hen’s egg, peanuts, tree nuts, soy, wheat, fish, shellfish, some fruits and vegetables
Some are heat-stable (survive cooking), others are destroyed by heat
→ Can cause oral allergy syndrome, gastrointestinal symptoms, skin reactions, or systemic reactions.

Contact allergens (via skin contact)

Metals (e.g. nickel in jewelry)
Cosmetics, fragrances, preservatives
Latex
Certain plants (e.g. poison ivy components)
→ Can cause allergic contact dermatitis (eczema-like skin inflammation).

Injectable or drug allergens

Components of insect venom (bee, wasp)
Certain medications (e.g. some antibiotics, anesthetics, contrast media)
→ Can cause local swelling or systemic reactions up to anaphylactic shock.

Sensitization: How an Allergy Develops

The first contact with an allergen in an allergic person usually does not cause strong symptoms. Instead, it primes the immune system—a process called sensitization. Major steps (simplified):

Uptake and presentation of allergen

Allergen is taken up by antigen-presenting cells (APCs), such as dendritic cells in skin or mucosa.
APCs migrate to regional lymph nodes and present allergen fragments to T helper cells.

Skewing toward an allergic response

In susceptible individuals, certain T helper cells develop into a subtype (often called Th2 cells).
These Th2 cells release cytokines that promote an “allergic-type” immune response.

Class switching of B cells

B cells that recognize the allergen, under the influence of Th2 cytokines, switch to producing IgE antibodies specific for that allergen.

Sensitization of effector cells

Allergen-specific IgE binds to high-affinity receptors on the surface of mast cells and basophils.
The individual is now sensitized: mast cells are “loaded” with IgE and ready to react to future allergen encounters.

This sensitization phase is usually silent. The noticeable allergic reaction comes with re-exposure.

Allergic Reaction: Immediate and Late Phases

When a sensitized person encounters the same allergen again, the reaction can proceed in characteristic phases.

Immediate Phase (Minutes)

Allergen binding and cross-linking

Allergen binds to IgE antibodies on mast cells.
Cross-linking of IgE molecules triggers mast cell activation and degranulation.

Release of mediators

Preformed mediators: e.g. histamine, some enzymes.
Newly formed mediators: leukotrienes, prostaglandins, various cytokines.

Effects of mediators

Vasodilation and increased blood vessel permeability → redness, swelling.
Smooth muscle contraction → bronchoconstriction in airways (wheezing, shortness of breath), cramps in gut.
Stimulation of nerves → itching, sneezing, coughing.

This is responsible for typical rapid symptoms such as itchy eyes, runny nose, wheals on the skin, or sudden breathing problems after exposure.

Late Phase (Hours)

Several hours after the immediate reaction:

Immune cells such as eosinophils, Th2 cells, and others are recruited to the tissue.
They release additional mediators and enzymes that sustain inflammation.
This can lead to prolonged swelling, mucus production, and tissue damage.

In chronic allergic diseases (e.g. persistent asthma, chronic eczema), the late and ongoing inflammatory phases are particularly important and can lead to lasting structural changes in tissues (remodeling of airways, thickening of skin).

Types of Allergic Diseases (Selected Examples)

Detailed disease descriptions belong in more specialized chapters; here only the typical patterns are outlined.

Allergic Rhinitis and Conjunctivitis

Often triggered by pollen, house dust mites, or animal dander.
Symptoms:

Sneezing fits, runny or stuffy nose
Itchy, watery, red eyes
Itching in throat and ears

Can be seasonal (pollen) or perennial (mites, pets).

Allergic Asthma

Chronic inflammation and hyperreactivity of the lower airways.
Symptoms:

Recurrent episodes of wheezing, shortness of breath, chest tightness, coughing.
Symptoms may be triggered by allergens, cold air, exercise, infections.

Airways narrow due to muscle contraction, swelling of mucosa, and mucus production.

Urticaria and Angioedema

Urticaria (hives): itchy, raised, red wheals on skin that appear and disappear quickly.
Angioedema: deeper swelling, often of eyelids, lips, tongue, or hands/feet; can be painful rather than itchy.
Triggers can include foods, medications, insect stings, physical stimuli (pressure, cold), or may be idiopathic.

Allergic Contact Dermatitis

Delayed-type skin reaction after contact with low-molecular-weight substances (haptens) that bind to skin proteins.
Typical of metal allergies (nickel), fragrances, preservatives, rubber chemicals.
Symptoms:

Redness, itching, small blisters, scaling in the contact area, often with a delay of 1–3 days.

Food Allergies

Can cause:

Itching and swelling in mouth (oral allergy syndrome)
Nausea, vomiting, abdominal cramps, diarrhea
Skin reactions (hives, eczema worsening)
Respiratory symptoms
Systemic reactions including anaphylaxis

Often begin in childhood but can also appear in adults.

Insect Venom Allergy

Mediated largely by IgE against bee or wasp venom components.
Local reactions (large swelling around sting) are common and usually harmless.
In sensitized individuals, systemic reactions can occur: hives, swelling far from sting, breathing difficulty, drop in blood pressure, up to anaphylactic shock.

Anaphylaxis: Systemic, Life-Threatening Allergy

Anaphylaxis is a severe, generalized allergic reaction that affects multiple organ systems simultaneously and can be life-threatening.

Typical features (not all must be present):

Skin: widespread hives, flushing, or angioedema.
Airways: hoarseness, swelling in throat, wheezing, shortness of breath.
Circulation: rapid pulse, drop in blood pressure, dizziness, collapse.
Gut: nausea, vomiting, diarrhea, abdominal cramps.

Common triggers:

Foods (e.g. peanuts, tree nuts, shellfish)
Insect venom
Medications
Less commonly, physical exercise together with certain foods or medications.

Rapid recognition and emergency treatment (e.g. intramuscular epinephrine, maintaining airway, emergency services) are crucial. People at risk often carry an emergency kit as part of their management plan.

Diagnostic Approaches in Allergies

Specific procedures and lab methods are treated in more detail elsewhere; here is the allergy-specific logic.

Medical History (Anamnesis)

Central tool: timing, context, and pattern of symptoms.
Questions often include:

What symptoms appear?
When do they appear (season, after meals, at work, with animals)?
How quickly after exposure?
Do medications (e.g. antihistamines) improve symptoms?

A careful history often narrows down likely allergens and guides testing.

Skin Tests

Prick test:

Small drops of allergen extracts are placed on the skin (usually forearm or back).
Skin is lightly pricked so allergen enters superficial layers.
After about 15–20 minutes, wheal and redness indicate sensitization.

Intradermal tests:

Allergen is injected into the skin in very small amounts.
More sensitive but with higher risk of stronger reactions; used selectively.

Patch test:

Used for suspected contact allergies.
Patches with allergens are applied to the skin (usually back) for 48 hours.
Skin is examined after 2–3 days for eczema-like reactions.

Skin tests show the presence of sensitization; interpretation must always consider clinical history because not all sensitizations cause symptoms.

Laboratory Tests

Specific IgE measurement:

Blood test to detect IgE antibodies against particular allergens.
Useful if skin tests are not possible or need confirmation.

Cell-based tests:

In specialized settings, activation of basophils or other cells by allergens may be assessed.

Again, results alone do not prove clinical allergy; they must be correlated with real-life reactions.

Provocation Tests

Controlled exposure to a suspected allergen (e.g. nasal, bronchial, or oral provocation in a medical setting).
Used when diagnosis remains unclear and benefits outweigh risks.
Requires careful monitoring due to risk of provoking significant symptoms.

Risk Factors and Development of Allergies

Why some people develop allergies while others do not involves both genetic and environmental factors.

Genetic Predisposition (Atopy)

Tendency to develop IgE-mediated allergies is called atopy.
Often runs in families: if both parents are atopic, the risk for the child is significantly increased.
Atopic individuals may develop a sequence of different allergies over time (the “atopic march”):

Atopic dermatitis in infancy
Later food allergy
Subsequently allergic rhinitis and/or asthma

Environmental Influences

A variety of environmental factors influence allergy risk, especially early in life:

Exposure to allergens:

High or continuous exposure to certain allergens (e.g. house dust mites, pet dander) may promote sensitization in susceptible individuals.
Some exposures may protect, depending on timing and dose.

Microbial environment and infections:

Patterns of microbial exposure in early childhood appear important for immune system “training”.
Certain hypotheses suggest that reduced exposure to diverse microbes may favor allergic responses; details are covered elsewhere.

Lifestyle and environmental factors:

Urban living, air pollution, tobacco smoke, and indoor climate may influence allergy risk and severity.
Diet and obesity may also modulate inflammatory responses.

Occupational exposures:

Repeated exposure to specific substances at work (e.g. flour dust in bakers, animal proteins in laboratory workers, chemicals in hairdressers) can lead to occupational allergies.

Principles of Allergy Management

Detailed therapeutic regimes and pharmacology are treated in other sections; here are the core strategies specific to allergies.

Allergen Avoidance (Elimination or Reduction)

Best but not always possible.
Examples:

Avoiding foods that provoke reactions.
Using mite-proof bedding covers, washing bedding hot, reducing indoor humidity.
Removing pets from the home for severe animal dander allergies.
Wearing protective equipment in occupational settings.

Avoidance can significantly reduce symptoms and medication needs, but may affect quality of life and is not always realistic.

Symptomatic Treatment

Medications do not cure the allergy itself but reduce symptoms:

Antihistamines:

Block histamine receptors.
Reduce itching, sneezing, runny nose, hives.

Glucocorticoids (cortisone derivatives):

Strong anti-inflammatory drugs; often used as nasal sprays or inhalers for long-term control.
Systemic use reserved for severe cases.

Bronchodilators:

Used in asthma to open airways (often in combination with anti-inflammatory therapy).

Other agents:

Leukotriene receptor antagonists, topical calcineurin inhibitors for skin, etc.

Choice and dosage depend on disease type, severity, and individual factors.

Allergen-Specific Immunotherapy (Desensitization)

This method aims to modify the underlying immune response:

The patient receives gradually increasing doses of the relevant allergen (subcutaneously by injection or sublingually under the tongue).
Over years, the immune system is “re-educated”:

IgE levels may decrease, other antibody classes (like IgG) may increase and act as “blocking antibodies”.
T cell responses shift away from the strong Th2 pattern.

Particularly effective for:

Allergic rhinitis to pollen, house dust mites.
Some forms of allergic asthma.
Insect venom allergies (where it can significantly reduce life-threatening reaction risk).

Immunotherapy is a long-term treatment requiring careful indication and medical supervision.

Emergency Preparedness

For individuals at risk of severe reactions (especially anaphylaxis):

Prescription of an emergency kit, often containing:

Adrenaline (epinephrine) auto-injector
Fast-acting antihistamine
Glucocorticoid preparation

Training in:

Recognizing early signs of severe reactions.
Correct use of the auto-injector.
Seeking immediate medical help.

Education of family members, teachers, or colleagues is often essential.

Impact of Allergies on Individuals and Society

Allergies are among the most common chronic conditions in many countries and can substantially affect:

Quality of life:

Sleep disturbance, fatigue, concentration problems.
Limitations in work, school performance, sports, and social activities.

Economic burden:

Direct costs: medications, doctor visits, hospitalizations.
Indirect costs: sick days, decreased productivity.

Long-term health:

Poorly controlled allergic inflammation can contribute to chronic conditions (e.g. persistent asthma).
Early and adequate management can reduce complications.

Understanding allergies as specific, often preventable or controllable misreactions of the immune system helps guide rational prevention, diagnosis, and treatment strategies, which are elaborated further in the subsequent sections on particular allergy types and their prophylaxis and therapy.