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Late-Phase Reaction – Causes, Symptoms & Treatment

A late-phase reaction is a delayed immune response that occurs hours to days after exposure to a triggering substance. It is commonly associated with allergies and hypersensitivity.

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Things worth knowing about "Late-Phase Reaction"

A late-phase reaction is a delayed immune response that occurs hours to days after exposure to a triggering substance. It is commonly associated with allergies and hypersensitivity.

What is a Late-Phase Reaction?

A late-phase reaction (also called a delayed hypersensitivity reaction) is an immune response of the body that does not occur immediately but develops with a time delay of several hours to several days after contact with a triggering substance. In contrast to the immediate reaction (Type I allergy), which occurs within minutes and is mediated by IgE antibodies, the late-phase reaction is typically driven by cellular immune mechanisms, primarily involving T-lymphocytes and macrophages.

Causes and Triggers

Late-phase reactions can be triggered by a wide variety of substances and stimuli. Common triggers include:

Allergens: Pollen, house dust mites, mold, pet dander, or food can provoke late-phase reactions in sensitized individuals.
Contact substances: Nickel, fragrances, preservatives, or latex components frequently cause contact allergies with delayed onset.
Medications: Certain drugs, particularly antibiotics such as penicillin, can cause late-phase reactions.
Infectious agents: Bacterial or viral antigens can trigger delayed immune responses, as seen in the tuberculin reaction.

Immunological Basis

The late-phase reaction corresponds immunologically to the Type IV hypersensitivity reaction as classified by Coombs and Gell. This reaction type is cell-mediated and is not driven by antibodies but by activated T-helper cells and macrophages. After initial exposure to the trigger (sensitization phase), renewed contact leads to an amplified inflammatory response. This delayed inflammation produces the characteristic symptoms.

Symptoms

Symptoms of a late-phase reaction vary depending on the affected organ or tissue:

Skin: Redness, swelling, itching, blister formation, or eczema (e.g., in contact allergy)
Airways: Worsening asthma symptoms such as coughing, shortness of breath, or wheezing
Eyes: Redness, tearing, or swelling of the conjunctiva
General: Fatigue, malaise, or mild fever in rare cases

Diagnosis

Diagnosis of a late-phase reaction involves several methods:

Patch test (epicutaneous test): Standard method for diagnosing contact allergies; the allergen is applied to the skin for 48 hours, and results are read after 48 to 72 hours.
Intradermal test with delayed reading: Injection of an allergen into the skin, evaluated after 24 to 72 hours.
Lymphocyte transformation test (LTT): A laboratory test to assess T-cell activation by specific substances.
Medical history and clinical examination: A thorough patient history is essential for correctly attributing symptoms to a specific exposure.

Treatment

Treatment of a late-phase reaction depends on the severity of symptoms and the causative substance:

Avoidance: Eliminating contact with the triggering allergen is the most important measure.
Corticosteroids: Anti-inflammatory medications (e.g., cortisone cream for eczema or systemic corticosteroids for severe reactions) effectively reduce symptoms.
Antihistamines: May relieve accompanying symptoms such as itching, although they are less effective in pure late-phase reactions than in immediate reactions.
Allergen immunotherapy (desensitization): For certain allergies, immunotherapy can improve long-term tolerance.
Moisturizing creams: Support the skin barrier in cases of eczematous skin changes.

Distinguishing Late-Phase from Immediate Reactions

While the immediate reaction (Type I allergy) occurs within seconds to 30 minutes of allergen contact, mediated by IgE antibodies, and can cause symptoms such as hives, angioedema, or anaphylactic shock, the late-phase reaction develops much more slowly. It is generally less life-threatening but can cause chronic complaints and persistent skin changes. In conditions such as bronchial asthma, both immediate and late-phase reactions may occur in sequence, a pattern known as a dual response.

When to See a Doctor

Medical attention should be sought in the following situations:

Severe or spreading skin symptoms that do not improve
Respiratory complaints or difficulty breathing
Recurring unexplained symptoms after specific exposures
Suspected drug hypersensitivity

References

Coombs RRA, Gell PGH: Classification of allergic reactions responsible for clinical hypersensitivity and disease. In: Gell PGH, Coombs RRA (eds.): Clinical Aspects of Immunology. Blackwell, Oxford, 1963.
Ring J, Beyer K, Biedermann T et al.: Guidelines for acute therapy and management of anaphylaxis. Allergo J Int. 2014;23:96-112.
Janeway CA, Travers P, Walport M, Shlomchik MJ: Immunobiology: The Immune System in Health and Disease. 9th ed. Garland Science, New York, 2016.

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