Immunology of Hypersensitivity

Summary

This session focuses on hypersensitivity and allergies, explaining the four types of hypersensitivity (Type I to Type IV), with emphasis on Type I (IgE-mediated) and Type IV (delayed cell-mediated). It covers the causes, symptoms, and treatments for allergic reactions, including food allergies, asthma, and anaphylaxis. The session discusses environmental and genetic factors contributing to allergies, common allergens, and diagnostic tests like skin prick tests and specific IgE blood tests. Treatment options, including adrenaline (EpiPen) for anaphylaxis and corticosteroids for Type IV hypersensitivity, are also highlighted, providing essential insights for managing allergic reactions.

Takeaways

• 😀 Hypersensitivity refers to an inappropriate or exaggerated immune response, often causing tissue damage.
• 😀 There are four types of hypersensitivity: Type 1 (IgE mediated), Type 2 (antibody-mediated), Type 3 (immune complex), and Type 4 (cell-mediated).
• 😀 Type 1 hypersensitivity is associated with conditions like anaphylaxis, asthma, and allergic rhinitis.
• 😀 Allergies involve the immune system responding to harmless substances, causing inflammation and dysfunction in organs.
• 😀 Allergic diseases have been increasing globally, with 30-40% of people affected, and asthma is the most common chronic disease among children.
• 😀 Food allergies can lead to severe reactions like anaphylaxis, with common allergens including nuts, eggs, and milk.
• 😀 The hygiene hypothesis suggests that environmental factors, such as a lack of exposure to infections in childhood, contribute to allergic diseases.
• 😀 Treatment for anaphylaxis involves adrenaline (EpiPen), which must be administered according to prescribed guidelines.
• 😀 Diagnosis of food allergies often includes skin prick tests, blood tests for specific IgE, and food challenges to identify the exact allergen.
• 😀 Type 2 hypersensitivity involves antibodies reacting with cell-bound antigens, leading to conditions like autoimmune diseases or allograft rejection.
• 😀 Type 4 hypersensitivity is cell-mediated and causes delayed reactions, often leading to chronic inflammation or conditions like allergic contact dermatitis and granulomas.

Q & A

What is hypersensitivity?

Hypersensitivity is an adaptive immune response that occurs in an inappropriate or exaggerated way, often resulting in tissue damage. It can be classified into four types based on the immune mechanisms involved.

What are the four types of hypersensitivity?

The four types of hypersensitivity are: Type 1 (IgE-mediated), Type 2 (antibody-mediated with cell-bound antigens), Type 3 (immune complex-mediated), and Type 4 (cell-mediated).

How is Type 1 hypersensitivity associated with allergic reactions?

Type 1 hypersensitivity is mediated by IgE antibodies and is associated with immediate allergic reactions, such as asthma and anaphylaxis, triggered by allergens like pollen, dust mites, or food.

What is the hygiene hypothesis in relation to allergies?

The hygiene hypothesis suggests that lack of early childhood exposure to infectious agents, microorganisms, and parasites can increase susceptibility to allergic diseases, as the immune system does not develop a proper tolerance to harmless substances.

What are common allergens that may cause an allergic reaction?

Common allergens include foods (e.g., nuts, milk, eggs), inhalants (e.g., pollen, mold), insect venoms, and substances like latex and certain medications.

How does anaphylaxis occur and what is its treatment?

Anaphylaxis is a severe allergic reaction that leads to shock, airway constriction, and swelling. It occurs when allergens enter the bloodstream, and the treatment is adrenaline (via an EpiPen) to counteract the effects.

What is the difference between Class 1 and Class 2 food allergies?

Class 1 food allergies typically occur in children and often involve allergens like milk and eggs, which are usually outgrown by age 3-6. Class 2 allergies are more common in adults and are associated with respiratory allergies and cross-reactivity between food and airborne allergens.

What are the key diagnostic tests for food allergies?

Key diagnostic tests for food allergies include skin prick tests, specific IgE blood tests, and food challenges, which may be open or double-blind, to identify the specific allergen causing the reaction.

How does the skin prick test work in diagnosing allergies?

The skin prick test involves injecting a small amount of allergen just under the skin and observing for swelling or redness within 30 minutes. It helps identify immediate allergic reactions and is operationally dependent on the quality of the allergens used.

What is Type 4 hypersensitivity, and how does it differ from other types?

Type 4 hypersensitivity is cell-mediated and delayed, occurring 24-72 hours after exposure. It involves T cells and macrophages, unlike other types, which are antibody-mediated. An example is allergic contact dermatitis, which can be diagnosed with a patch test.