Hypersensitivity Reactions Overview
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Questions and Answers

Which of the following is NOT a localized reaction associated with allergic reactions?

  • Asthma
  • Allergic rhinitis
  • Pneumonia (correct)
  • Urticaria

    • What is another term used to describe allergic reactions?

  • Autoimmunity
  • Anaphylaxis
  • Immunodeficiency
  • Atopic reactions (correct)

    • What type of antigens are involved in allergic reactions?

  • Endogenous antigens
  • Exogenous antigens (correct)
  • Autoantigens
  • All of the above

    • Why do allergic reactions not typically cause autoimmune diseases?

    <p>Allergic reactions target exogenous antigens, not the body's own tissues. (C)</p> Signup and view all the answers

    Which of the following statements about allergic reactions is TRUE?

    <p>They can be delayed for up to 24 hours. (A)</p> Signup and view all the answers

    Which type of hypersensitivity is known as cytotoxic hypersensitivity?

    <p>Type II (B)</p> Signup and view all the answers

    Which of the following descriptions best fits Type I hypersensitivity?

    <p>Allergy or immediate hypersensitivity (C)</p> Signup and view all the answers

    What does the acronym ACID stand for in reference to hypersensitivity types?

    <p>Atopy, Cytotoxic, Immune complex, Delayed (A)</p> Signup and view all the answers

    Which type of hypersensitivity involves immune complex mechanisms?

    <p>Type III (D)</p> Signup and view all the answers

    Among the following, which type does not fit the definition of immediate hypersensitivity?

    <p>Type IV (A)</p> Signup and view all the answers

    Flashcards

    Type I Hypersensitivity

    Allergy or immediate hypersensitivity reaction, often triggered by allergens.

    Type II Hypersensitivity

    Cytotoxic hypersensitivity where antibodies attack own cells, often seen in autoimmune diseases.

    Type III Hypersensitivity

    Immune complex hypersensitivity, where antigen-antibody complexes deposit in tissues triggering inflammation.

    Hypersensitivity Types

    Classified by the mechanism of action into four types: I, II, III, and IV.

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    Immediate Hypersensitivity

    Reactions that occur quickly after exposure to an allergen, characteristic of Type I hypersensitivity.

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    Atopy

    A genetic tendency to develop allergic diseases such as asthma or rhinitis.

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    Exogenous Antigens

    Substances from outside the body that trigger allergic reactions.

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    Localized Allergy Response

    An allergic reaction that occurs in a specific area of the body.

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    Examples of Localized Reactions

    Common localized allergic reactions include urticaria, allergic rhinitis, and asthma.

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    Study Notes

    Hypersensitivity Reactions

    • Hypersensitivity reactions are exaggerated immune responses to exogenous or endogenous (self) antigens.
    • Antigens can include infections (bacterial, viral, fungal), harmless substances (animals' wool, drugs, plant pollens, metals, dust, foods like eggs, fruits, peanuts, seafood), and self-antigens (autoimmunity).
    • Stages of hypersensitivity reactions include sensitization (asymptomatic first contact with an antigen) and effector (symptomatic phase during second and repeated exposures).

    Types of Hypersensitivity

    • There are four types of hypersensitivity reactions (ACID):
      • Type I (Allergy, atopy, or immediate hypersensitivity)
      • Type II (Cytotoxic hypersensitivity)
      • Type III (Immune complex hypersensitivity)
      • Type IV (Delayed or cell-mediated hypersensitivity)
    • Types I, II, and III are antibody-mediated, while type IV is cell-mediated.

    Type I Hypersensitivity (Immediate Hypersensitivity)

    • Symptoms appear 15-30 minutes (or up to 6-24 hours) after allergen exposure.
    • Type I is also known as allergy or atopy; it does not cause autoimmune diseases.
    • Examples:
      • Localized reactions: urticaria, allergic rhinitis, asthma, allergic conjunctivitis.
      • Systemic reaction (anaphylaxis): low blood pressure and airway obstruction, can be fatal
    • Mechanism (First exposure - Sensitization phase): Plasma cells produce specific IgE, which binds to Fce receptors on mast cells and basophils.
    • Mechanism (Second and repeated exposure - Early reaction): Allergen binds to specific IgE on mast cells, leading to degranulation and release of inflammatory mediators (tryptase, histamine, prostaglandins).
      • Effects of mediators: itching, vasodilation, increased vascular permeability, edema, erythema, hypotension, contraction of bronchial and intestinal muscles. (early reaction).
    • Mechanism (Late reaction): Attraction of eosinophils results in tissue damage.

    Type II Hypersensitivity (Cytotoxic Hypersensitivity)

    • This type leads to cell destruction.
    • Antigens: Exogenous (drug bound to cell membrane) or Endogenous (part of cell membrane -- autoimmune disease).
    • Mechanism: IgM or IgG bind to the cell membrane, leading to cell destruction through:
      • Opsonization and phagocytosis
      • Natural killer (NK) cell activation
      • Complement activation and cell lysis by MAC (membrane attack complex).
    • Clinical examples:
      • Blood transfusion reaction (anti-A/B antibodies)
      • Hemolytic disease of the fetus and newborn (Rh incompatibility)
      • Autoimmune diseases (anti-acetylcholine receptor in myasthenia gravis, drug-induced autoimmune hemolytic anemia, like penicillin)

    Type III Hypersensitivity (Immune Complex Hypersensitivity)

    • Known as immune complex hypersensitivity.
    • Soluble antigens form immune complexes.
    • Antigens: Exogenous (drugs) or Endogenous (soluble self-antigen in autoimmune diseases).
    • Mechanism: IgG antibodies bind to circulating antigens, forming immune complexes that deposit in tissues, especially blood vessels. They activate complement and neutrophils leading to inflammation (e.g., vasculitis).
    • Clinical examples:
      • Arthus reaction (localized in skin, like from vaccines)
      • Serum sickness (systemic reaction from injected drugs)
      • Systemic lupus erythematosus (SLE) (antibodies against DNA, RNA, histones)

    Type IV Hypersensitivity (Delayed or Cell-Mediated Hypersensitivity)

    • This type is cell-mediated, peaking 48-72 hours after exposure to antigens.
    • Antigens: Intracellular pathogens (e.g., mycobacteria, viruses), metals, chemicals, and self-antigens.
    • Cells involved: CD4+ T helper-1 cells, macrophages, and CD8+ T cytotoxic cells.
    • Mechanism: Sensitized Th1 cells release cytokines (like IFN-γ), attracting macrophages and activating them, or directly damaging cells.
    • Clinical examples:
      • Contact dermatitis (e.g., nickel, cosmetics, plants like ivy)
      • Granulomatous formation (e.g., tuberculosis)
      • Tuberculin skin test
      • Some autoimmune diseases (e.g., type 1 diabetes, multiple sclerosis)
      • Drug-induced rash (e.g., after amoxicillin)

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    Hypersensitivity Reactions (Immunology) PDF

    Description

    This quiz explores the various types of hypersensitivity reactions, focusing on their mechanisms, stages, and the immune responses involved. Understand the differences between Type I, II, III, and IV hypersensitivity, and the role of antigens in triggering these responses. Enhance your knowledge of immune system behaviors and allergies.

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