10 Hypersensitivity Reactions

Objectives of the Lecture

• Understand allergic reactions, specifically hypersensitivity type I, and analyze their protective mechanisms against parasitic infections by illustrating how IgE antibodies play a role in defense mechanisms.

• Describe effector mechanisms in IgE-mediated allergic reactions, focusing on the roles played by mast cells, basophils, and eosinophils, as well as the release of mediators like histamines.

• Identify different manifestations of allergic diseases, highlighting the symptoms and impacts of common allergies such as food allergies, allergic rhinitis, and asthma.

• Explore the interaction between various environmental factors (including urbanization and sanitation practices) and genetic susceptibility, emphasizing the hygiene hypothesis that correlates reduced exposure to microbes in early life with increased allergy prevalence.

• Understand non-IgE-mediated hypersensitivity reactions, detailing different types and their mechanisms as well as their clinical relevance.

Immunology of the Allergic Response

• Definition: The immune system's response to external substances, referred to as antigens or allergens, can lead to harmful allergic diseases that affect the quality of life.

• Distinction between "allergy" (uncommitted biological response to an allergen) and "allergic disease" (a harmful and often chronic response leading to symptoms) is critical for understanding patient experiences. Notably, the traditional understanding of "allergy" is confined to specific IgE-mediated responses, while the broader category of hypersensitivity includes other immune mechanisms.

History of Allergy Research

• 1919: The initial identification of reaginic activity leading to asthma symptoms following blood transfusions marked a significant milestone in allergy research.

• 1921: The work of Prausnitz and Küstner demonstrated the presence of a tissue-sensitizing antibody known as reagin, which is now recognized as immunoglobulin E (IgE). This discovery provided a clear link between specific IgE responses and allergic reactions.

• 1966: Gell and Coombs introduced their classification of hypersensitivity reactions that enabled clearer categorization of allergic responses based on immune mechanisms, enhancing the understanding of disease processes.

Gell and Coombs Classification of Hypersensitivity Reactions

• Types of Reactions: This framework categorizes allergic responses based on immune effectors, including antibodies and T cells.

  • Type I: IgE-mediated, often leading to rapid allergic responses such as anaphylaxis and common allergies (e.g., hay fever, food allergies).

  • Type II: IgG-mediated cytotoxicity which occurs in conditions such as hemolytic anemia and transfusion reactions.

  • Type III: Immune complex-mediated responses that results in conditions like serum sickness and some forms of vasculitis.

  • Type IV: T cell-mediated (delayed-type hypersensitivity) responses seen in contact dermatitis and graft-versus-host disease. These reactions can occur hours to days after exposure to the allergen.

Atopy and IgE-Mediated Responses

• Atopy: A genetic predisposition to produce IgE antibodies against environmental antigens, linking to allergic conditions that are characterized by elevated IgE levels, with frequent conditions including allergic rhinitis, atopic dermatitis, and asthma.

• Type 2 Immune Responses: These immune responses protect against parasitic infections through mechanisms like increased mucus production and fluid secretion, helping to trap, expel, and eliminate harmful organisms effectively.

Common IgE-Mediated Reactions

• Reactions: Symptoms can develop immediately following allergen exposure; common manifestations include local swelling (wheal) or systemic reactions, such as anaphylaxis, a potentially life-threatening reaction.

• Common allergens for systemic effects include various foods (e.g., peanuts, tree nuts), medications (e.g., penicillin), insect venoms (e.g., bee stings), dust mites, and pollen, which are known to trigger allergic responses with varying severities.

• Local Reactions: Often involve symptoms in the skin (urticaria, eczema), respiratory system (asthma characterized by wheezing and difficulty breathing), and digestive tract (symptoms of food allergies can consist of nausea, vomiting, and abdominal pain).

Mechanisms of Allergic Reactions

• Sensitization Step: The initial exposure to an allergen is crucial for the activation of the immune system, leading to IgE production. Upon subsequent exposure, these pre-existing IgE antibodies bind to allergens, culminating in the development of allergic symptoms.

• Mast Cells: These are the key effector cells in allergic responses. Mucosal mast cells reside in respiratory and intestinal tissues, while connective tissue mast cells are found in the skin and other tissues.

  • Activation: Upon allergen binding to IgE on mast cells via high-affinity receptors (FcεRI), mast cells release mediators like histamine, leukotrienes, and prostaglandins that cause inflammation and allergy symptoms, which can lead to increased vascular permeability and bronchial smooth muscle contraction.

• Basophils and Eosinophils: Both cell types amplify allergic responses; basophils release cytokines that sustain inflammation, while eosinophils contribute to tissue damage and inflammation through the release of cytotoxic granules and mediators.

Impacts of Mast Cell Activation

• Tissue-specific effects of mast cell activation may include:

  • Gastrointestinal: Results in symptoms like increased fluid secretion, diarrhea, and vomiting, which can lead to dehydration.

  • Respiratory: Characterized by increased mucus production, bronchial constriction, and asthma symptoms like wheezing and coughing.

  • Skin: Leads to swelling, redness, and conditions such as urticaria or atopic dermatitis.

• Chronic activation of mast cells can result in airway remodeling in asthma due to persistent inflammation; this can lead to irreversible changes in airway structure and function over time.

The Hygiene Hypothesis

• This hypothesis suggests that the higher prevalence of allergic diseases in Western societies correlates with reduced microbial exposure in early life, which impacts the development of the immune system and regulatory T cell populations responsible for modulating immune responses.

• Environments with a diverse exposure to microbes are thought to promote protective immune responses against allergies, supporting healthier immune system development and balance.

Non-IgE-Mediated Hypersensitivity

• Non-IgE-mediated hypersensitivity reactions recognize other forms of immune responses.

  • Type II: These IgG-mediated reactions may manifest in autoimmune conditions where the body's immune system targets its own cells.

  • Type III: Immune complex-mediated conditions can manifest as serum sickness, which involves immune complexes depositing in tissues and causing inflammation.

  • Type IV: Delayed-type hypersensitivity mediated by T cells can lead to responses seen in conditions such as contact dermatitis and the rejection of transplanted organs, demonstrating the role of cellular immunity in allergic responses.

Example: Celiac Disease

• Celiac disease is characterized as a type IV hypersensitivity and an autoimmune disorder triggered by gluten ingestion, specifically in genetically susceptible individuals. It can lead to villous atrophy and malabsorption in the small intestine, resulting in various gastrointestinal symptoms.

• Though not directly IgE-mediated, it exemplifies complex immune responses, displaying similarities to typical allergic reactions where the immune system reacts adversely to certain substances.

Treatments for Allergic Diseases

• Conservative treatment approaches include the use of antihistamines to alleviate symptoms, corticosteroids to reduce inflammation, and desensitization therapies, such as allergen immunotherapy, that aim to modify the immune response over time.

• Targeted therapies have emerged, including the use of anti-IgE monoclonal antibodies, which have shown efficacy in controlling severe allergic responses, particularly in patients with chronic asthma or allergic rhinitis.

Conclusion

• Understanding the underlying mechanisms of hypersensitivity, along with their interaction with genetics and environmental factors, is critical for effectively managing and treating allergic diseases. Ongoing research into these interactions continues to inform treatment strategies and improve patient outcomes in the field of allergy and immunology.