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Sensitization – Causes, Mechanism and Treatment

Sensitization is the process by which the immune system develops a specific response to a substance, making the body susceptible to allergic reactions upon future exposure.

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Sensitization is the process by which the immune system develops a specific response to a substance, making the body susceptible to allergic reactions upon future exposure.

What is Sensitization?

Sensitization is a medical term that describes the initial process through which the immune system is exposed to a foreign substance – known as an allergen – and mounts a specific immune response. Once sensitized, the immune system remembers the allergen, and subsequent exposures can trigger an exaggerated immune response known as an allergic reaction. Sensitization itself typically causes no symptoms, but it sets the stage for future allergic disease.

Causes and Triggers

A wide range of substances can cause sensitization. The most common allergens include:

Inhalant allergens: Pollen, house dust mites, animal dander, mold spores
Food allergens: Peanuts, tree nuts, milk, eggs, wheat, fish, shellfish
Contact allergens: Nickel, latex, fragrances, preservatives
Medications: Antibiotics such as penicillin, non-steroidal anti-inflammatory drugs
Insect venoms: Bee and wasp venom

Genetic predisposition (atopy), environmental factors, and an overactive or dysregulated immune system all increase the likelihood of sensitization occurring.

Mechanism of Sensitization

During first-time allergen exposure, antigen-presenting cells capture the allergen and present it to T lymphocytes. This activates B lymphocytes, which produce allergen-specific immunoglobulin E (IgE) antibodies. These IgE antibodies bind to mast cells and basophils throughout the body. Upon re-exposure to the allergen, it binds to the IgE antibodies on these cells, triggering the release of inflammatory mediators such as histamine, which causes the symptoms of an allergic reaction.

Types of Sensitization

Type I Sensitization (Immediate Hypersensitivity)

This is the most common form of sensitization and is mediated by IgE antibodies. It underlies classic allergic conditions such as hay fever, allergic asthma, food allergy, and life-threatening anaphylaxis.

Type IV Sensitization (Delayed-Type Hypersensitivity)

In delayed-type hypersensitivity, T lymphocytes rather than antibodies mediate the immune response. Reactions typically occur 24 to 72 hours after allergen contact. A classic example is allergic contact dermatitis, such as reactions to nickel or certain cosmetic ingredients.

Symptoms Following Sensitization

Once sensitized, re-exposure to the allergen can trigger a variety of symptoms depending on the route of exposure and the organ system involved:

Skin reactions: Redness, itching, hives, eczema
Eye reactions: Itching, tearing, conjunctivitis
Respiratory reactions: Sneezing, runny nose, asthma attacks, shortness of breath
Gastrointestinal reactions: Nausea, vomiting, diarrhea, abdominal cramps
Systemic reactions: Drop in blood pressure, circulatory failure (anaphylaxis)

Diagnosis

Sensitization is diagnosed by an allergist or immunologist using the following methods:

Skin prick test: Small amounts of allergen extracts are applied to the skin and lightly pricked through.
Intradermal test: Allergens are injected directly into the skin for more sensitive detection.
Patch test (epicutaneous test): Allergens are applied to the skin under a patch for 48 to 72 hours to detect delayed-type reactions.
Blood test (specific IgE): Measurement of allergen-specific IgE antibodies in the blood using RAST or ImmunoCAP assays.

It is important to note that a positive sensitization test does not automatically mean that a clinically relevant allergy is present. Results must always be interpreted in the context of the full clinical picture.

Treatment and Prevention

Allergen Avoidance

The most effective strategy is to avoid exposure to the triggering allergen wherever possible.

Pharmacological Treatment

Medications used to manage allergic symptoms include:

Antihistamines to relieve itching, sneezing, and nasal symptoms
Corticosteroids to reduce inflammation
Bronchodilators for asthma-related symptoms
Epinephrine (adrenaline) as emergency treatment for severe anaphylactic reactions

Allergen Immunotherapy (Desensitization)

Allergen-specific immunotherapy (AIT), also known as desensitization or hyposensitization, is the only disease-modifying treatment available. It involves administering gradually increasing doses of the allergen either by subcutaneous injection (subcutaneous immunotherapy, SCIT) or under the tongue (sublingual immunotherapy, SLIT). The goal is to retrain the immune system to tolerate the allergen and permanently reduce hypersensitivity.

References

Janeway, C.A. et al. - Immunobiology: The Immune System in Health and Disease. Garland Science, 9th Edition (2016).
World Allergy Organization (WAO) - White Book on Allergy, Update 2013. Available at: https://www.worldallergy.org
Muraro, A. et al. - EAACI Guidelines on Allergen Immunotherapy. Allergy, European Academy of Allergy and Clinical Immunology (2018). Available at: https://www.eaaci.org

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