Hypersensitivity Reactions
Also written as HSR.
Co-trimoxazole is contraindicated in severe hypersensitivity reactions.
Hypersensitivity Reactions
- Hypersensitivity reactions are exaggerated or inappropriate immune responses that cause tissue damage. They are classified into four main types:
- Type I (Immediate) Hypersensitivity is IgE mediated . It involves the rapid degranulation of mast cells upon re-exposure to an allergen, leading to the release of inflammatory mediators. Examples include anaphylaxis and asthma .
- Type II (Cytotoxic) Hypersensitivity is mediated by IgG or IgM antibodies that bind to antigens on cell surfaces. This binding can lead to cell destruction through complement activation or antibody-dependent cell-mediated cytotoxicity (ADCC
- Type III (Immune Complex) Hypersensitivity occurs when antigen-antibody complexes form in the circulation and deposit in various tissues, such as blood vessel walls, glomeruli, or joints. These deposits trigger inflammation and tissue damag
- Type IV (Delayed-Type) Hypersensitivity is unique as it is T lymphocyte mediated , primarily by Th1 cells , rather than antibodies. The reaction typically develops 24–72 hours after antigen exposure. Examples include the tuberculin test and
Read this in Cellular Immunology Short Answer Questions — Notes
Hypersensitivity Reactions
- Chronic antigen exposure → Urticaria .
- Massive cyst rupture → Severe anaphylaxis (can be fatal).
Read this in Cestodes (Tapeworms) — Notes
What our notes say
- Co-trimoxazole is contraindicated in severe hypersensitivity reactions. (Pneumocystis jirovecii pneumonia (PCP))
- patients (BT, BB, BL) and are delayed hypersensitivity reactions. (Davidson's Essentials of Medicine - 2nd)
- 312), intracavitary\naspergilloma or invasive pulmonary aspergillosis.\nAllergic bronchopulmonary aspergillosis\nABPA is a hypersensitivity reaction to A. fumigatus. (Davidson's Essentials of Medicine - 2nd)
- \n\n 132 caused by the expectoration and microscopic visualiza- Hypersensitivity reactions usually require the discon tion of dead bacilli. (Harrisons Pulmonary and Critical Care Medicine)
- Increased capillary permeability also may sterone secretion elevated but the biologic half-life of\nbe a consequence of a hypersensitivity reaction and is aldosterone is prolonged, which increases further the\ncharacteristic of immune injury. (Harrisons Cardiovascular Medicine 2nd Edition 2)
- It is mainly a type III hypersensitivity reaction,\nbut with more chronic exposure to the antigen, there may be a component of type IV hypersensitivity with\ngranulomatous inflammation and fibrosis. (3 - Summary and MCQs)
Study material on Hypersensitivity Reactions
- 5-Hypersensitivity Reactions
- Harrisons Pulmonary and Critical Care Medicine
- Davidson's Essentials of Medicine - 2nd
- Kenya-ARV-Guidelines-2022-2
- Kenya-ARV-Guidelines-2022-2
- Immunopathology complete MCQs
- Immunopathology
- Basic Pharmacology Notes
- Devendra K. Gupta, Krishna M. Goel - Hutchison’s Paediatrics. chdhe-Jaypee Brothers (2012)
- Immunopathology Cat: Hypersensitivity & Anaphylaxis MCQs — CAT
- Weekly Year 2: Cellular Immunology Exam - March 27, 2026 (Section A: MCQs)
Practice questions on Hypersensitivity Reactions
A positive tuberculin skin test, with a firm 15 mm diameter dark red area of induration on the forearm, appears 60 hours after injection of purified protein derivative (PPD). This finding is most likely a consequence of which type of hypersensitivity reaction?
- Type I hypersensitivity
- Type II hypersensitivity
- Type IV hypersensitivity
- Type III hypersensitivity
- Complement activation
Answer: C. * The tuberculin skin test relies on an intact cell-mediated immune response. It is positive with prior mycobacterial exposure, so memory T cells are present and can be reactivated.
A positive tuberculin skin test shows 15 mm of firm dark red induration at 60 hours after PPD injection. Most likely type of hypersensitivity reaction?
- Complement activation
- Type I hypersensitivity
- Type IV hypersensitivity
- Type II hypersensitivity
Answer: C. Delayed reaction peaking at 48–72 hours + induration (not just redness) + requires prior sensitization = type IV (cell-mediated/delayed-type) hypersensitivity, mediated by memory T lymphocytes responding to mycobacterial antigens.
Which type of hypersensitivity reaction is mediated by IgE antibodies and involves mast cell degranulation and histamine release?
- Type I (Immediate)
- Type II (Antibody-mediated)
- Type III (Immune complex-mediated)
- Type IV (Delayed-type)
Answer: A. Type I hypersensitivity (immediate hypersensitivity) is mediated by IgE antibodies binding to mast cells and basophils, leading to rapid degranulation upon re-exposure to the allergen and release of inflammatory mediators like histamine.
Which immunoglobulin is associated with immediate hypersensitivity reactions and defense against parasitic infections, primarily by binding to Fc receptors on mast cells and basophils?
- IgM
- IgA
- IgG
- IgE
Answer: D. IgE antibodies are strongly associated with allergic reactions (Type I hypersensitivity) and play a role in immunity against parasites. They bind with high affinity to Fc receptors (FcεRI) on mast cells and basophils.