Hypersensitivity

• What is hypersensitivity?

  It is an exaggerated response to a typically harmless antigen that results in injury to the tissue, disease, or even death.
• What are the four types of hypersensitivity?

  1. Type 1 Hypersensitivity – Anaphylactic Hypersensitivity
  2. Type 2 Hypersensitivity – Antibody-Mediated Cytotoxic Hypersensitivity
  3. Type 3 Hypersensitivity – Complex Mediated Hypersensitivity
  4. Type 4 Hypersensitivity – Cell Mediated Hypersensitivity
• How are Types I, II, and III hypersensitivity classified?

  They are classified as immediate hypersensitivity, with symptoms developing within minutes to hours after exposure to the antigen.
• What is the classification of Type IV hypersensitivity?

  Type IV hypersensitivity is classified as delayed hypersensitivity, with symptoms manifesting 48-72 hours after exposure.
• What is the primary antibody involved in Type I hypersensitivity?

  IgE is the primary antibody involved in Type I hypersensitivity.
• What are allergens?

  Allergens are antigens that trigger Type I hypersensitivity and can be foreign substances.
• What does atopy refer to?

  Atopy refers to an inherited tendency to respond to naturally occurring inhaled and ingested allergens with continued production of IgE.
• What happens during the sensitization phase of Type I hypersensitivity?

  During the sensitization phase, the body is exposed to an allergen, and APCs present the antigen to Th2 cells, which produce cytokines that stimulate IgE production.
• What occurs during the activation phase of Type I hypersensitivity?

  During the activation phase, allergen-specific IgE binds to mast cells, leading to degranulation and the release of mediators like histamine.
• What is the most abundant mediator in the activation phase of Type I hypersensitivity?

  Histamine is the most abundant mediator in the activation phase.
• What are the clinical manifestations of Type I hypersensitivity?

  • Rhinitis: sneezing, runny nose
  • Asthma: caused by inhalation of small particles
  • Food Allergies
  • Dermatitis: Wheal and Flare reaction
• What is the treatment for systemic anaphylaxis?

  Epinephrine is administered in cases of systemic anaphylaxis.
• What is the role of anti-IgE monoclonal antibody (Omalizumab) in treating Type I hypersensitivity?

  It prevents circulating IgE from binding to mast cells and basophils.
• What is Dupilumab (Dupixent) used for?

  Dupilumab is used for the treatment of moderate to severe atopic dermatitis.
• What is allergy immunotherapy (AIT)?

  • Induces immune tolerance to a specific antigen
  • Gradually increases doses of allergen under physician guidance
• What are the in vivo skin tests for Type I hypersensitivity?

  • Direct skin testing (Percutaneous and Intradermal tests)
  • Percutaneous: Prick or puncture test
  • Intradermal test: more antigen is used
• What is the Radioallergosorbent Test (RAST)?

  RAST is a test performed to measure specific IgE by incubating the patient's serum and adding a radio-labeled antibody.
• What is the purpose of the Direct Antiglobulin Test (DAT)?

  DAT detects transfusion reactions, hemolytic disease of newborns, and autoimmune hemolytic anemia.
• What is Type II hypersensitivity?

  • Involves IgG and IgM antibodies against antigens on cell surfaces
  • Can lead to cellular destruction, cell function inhibition, and abnormal increase in cell function
• What is a common clinical example of Type II hypersensitivity?

  Acute hemolytic transfusion reactions occur within minutes to hours after receiving incompatible blood.
• What is the hemolytic disease of the newborn?

  It appears in infants whose mothers have been exposed to blood-group antigens on the baby's cells that differ from their own.
• What is the treatment for hemolytic disease of the newborn?

  Mothers are given Anti-D Immune Globulin (Rhogam) starting from 28 weeks of gestation.
• What is Type III hypersensitivity?

  • Involves IgG or IgM and is complement-mediated
  • Antigen is soluble, leading to tissue destruction
• What is the Arthus reaction?

  The Arthus reaction is a necrotic dermal reaction considered to be a local immune complex deposition phenomenon.
• What is serum sickness?

  Serum sickness results from passive immunization of humans with animal serum.
• What is Type IV hypersensitivity?

  • Involves sensitized T-cells rather than antibodies
  • It is a delayed type of hypersensitivity
• What is contact dermatitis?

  Contact dermatitis produces skin eruptions characterized by erythema, swelling, and papules appearing from 6 hours to several days after exposure.
• What is hypersensitivity pneumonitis?

  Hypersensitivity pneumonitis is an allergic disease of the lung characterized by inflammation of the alveoli and interstitial spaces.
• What are the types of lung diseases associated with hypersensitivity pneumonitis?

  • Farmer’s Lung Disease
  • Bird Breeder’s Lung Disease
  • Humidifier or Air Conditioner Lung Disease
• What is the gold standard for testing contact dermatitis?

  Patch test is the gold standard for testing contact dermatitis.
• What is the Mantoux test used for?

  The Mantoux test uses M. tuberculosis antigen extract to test for tuberculosis exposure.
• What is the procedure for the Mantoux test?

  0.1 mL of PPD is injected intradermally, and the reaction is checked within 48-72 hours.
• What indicates a positive Mantoux test?

  An induration of 15 mm or more indicates a positive Mantoux test.
• Actions: Smooth muscle contractions, vasodilation, increased vascular permeability

  Mediator: Histamine
• Mediator: Heparin

  Action: Smooth muscle contraction, vasodilation, increased vascular permeability
• Actions: Chemotactic for eosinophils

  Mediator: Eosinophil chemotactic factor of anaphylaxis (ECF-A)
• Action: Chemotactic for neutrophils

  Mediator: Neutrophil chemotactic factor of anaphylaxis (NCF-A)
• Action: Convert C3 to C3b, stimulate mucus production, activate cytokines

  Mediator: Proteases (e.g., tryptase, chymase)
• Actions: Vasodilations, increased vascular permeability

  Mediator: Prostaglandin PGD2
• Actions: Chemotactic for neutrophils and eosinophils

  Mediator: Leukotriene LTB4