Question

Hypersensitivity - I is associated with what type of immune complex? (Note: Type I Hypersensitivity is immediate hypersensitivity / allergy.)

• A. IgM antibody + Protein antigen complex
• B. IgG antibody + Protein antigen complex
• C. IgE antibody + Allergen complex
• D. IgA antibody + antigen complex

Hint:

• Type I (Allergy/Anaphylaxis) IgE-mediated, mast cell degranulation (Histamine). Allergen binds to IgE on mast cells.
• Type II (Cytotoxic) IgG/IgM against cell surface/matrix antigens. Complement activation, phagocytosis.
• Type III (Immune Complex) IgG/IgM immune complexes deposit in tissues. Complement, inflammation.
• Type IV (Delayed-Type) T cell-mediated (T\(_H\)1, CTLs). Cytokines, inflammation. No antibodies involved directly.

Answer 1

The Correct Option is C

Hypersensitivity reactions are exaggerated or inappropriate immune responses to antigens that can cause tissue damage. They are classified into four main types (Gell and Coombs classification):

• Type I Hypersensitivity (Immediate or Allergic Hypersensitivity): This type is mediated by IgE antibodies. Mechanism: 1. Initial exposure to an allergen (an antigen that causes allergy) leads to sensitization, where IgE antibodies specific to the allergen are produced and bind to Fc receptors on the surface of mast cells and basophils. 2. Upon re-exposure to the same allergen, the allergen cross-links the IgE antibodies bound to mast cells/basophils. 3. This cross-linking triggers the degranulation of these cells, releasing pre-formed mediators (like histamine, tryptase) and newly synthesized mediators (like prostaglandins, leukotrienes). 4. These mediators cause the symptoms of allergy (e.g., vasodilation, smooth muscle contraction, mucus secretion), such as in hay fever, asthma, food allergies, anaphylaxis. So, Type I involves "IgE antibody + Allergen complex" triggering mast cell degranulation.

Let's look at other types briefly for context:

• Type II Hypersensitivity (Antibody-mediated Cytotoxic Hypersensitivity): Mediated by IgG or IgM antibodies directed against antigens on cell surfaces or in the extracellular matrix. Leads to cell destruction (e.g., autoimmune hemolytic anemia, transfusion reactions). Forms antibody-cell surface antigen complexes.
• Type III Hypersensitivity (Immune Complex-mediated Hypersensitivity): Mediated by the formation of antigen-antibody (immune) complexes (usually IgG or IgM with soluble antigens) that deposit in tissues (e.g., blood vessels, kidneys, joints) and activate complement and inflammatory responses. (e.g., Serum sickness, Arthus reaction, some forms of glomerulonephritis).
• Type IV Hypersensitivity (Cell-mediated or Delayed-Type Hypersensitivity - DTH): Mediated by T lymphocytes (specifically T\(_H\)1 cells and cytotoxic T lymphocytes - CTLs), not antibodies. Takes 24-72 hours to develop. (e.g., Tuberculin skin test reaction, contact dermatitis like poison ivy, graft rejection).

The question asks about Type I Hypersensitivity. This is associated with IgE antibodies reacting with an allergen. Option (c) "IgE antibody + Allergen complex" correctly describes the key components involved in triggering Type I hypersensitivity. \[ \boxed{\text{IgE antibody + Allergen complex}} \]