Source:
McCance & Huether, 2020; Galli & Tsai, 2012.

Primary Exposure: Immunologic Memory

A type I hypersensitivity reaction requires sensitization against an allergen (environmental antigen) prior to producing an anaphylactic reaction. Sensitization is asymptomatic and occurs during the initial antigen-specific exposure. The initial exposure produces antigen-specific memory B and T cells.

Subsequent Exposure: Immediate Hypersensitivity

Sensitized mast cells degranulate, release histamine (+inflammatory mediators) and produce a range of clinical manifestations that vary in severity.

Onset: Immediate, within minutes (up to a few hours)

Physiological Effects:

Tissues that contain numerous mast cells are the most affected: GI, skin, respiratory.

Effects are produced through histamine 1 receptors (H1):

Vasodilation, increased capillary permeability, smooth muscle spasm, mucus secretions, edema.

Source:

Rogers, Julia . MCCANCE & HUETHER’S PATHOPHYSIOLOGY. 9th ed., Elsevier , pp. 255–63, https://pageburstls.elsevier.com/#/books/9780323789905. Accessed 11 Oct. 2025.

-higher production of IgE

-More Fc receptors (for IgE) on mast cells

-Cytokine (IL-4, IL-3) polymorphism

(Cytokines regulate IgE synthesis)

Family clustering:

1 parent has an allergy -> 40% incidence risk in offspring

BOTH parents have allergies -> up to 80% risk in offspring

Microbiome & Food Allergies:

The microbiota interacts with food antigens and impact immunogenicity. Dysbiosis (imbalance) in the microbiota is linked to activation of B cells, IgE ( in atopic individuals).

Thus, factors such as diet, drugs and comorbidities

have an impact on allergy response. A diet that is low-fiber/high-fat and drugs such as antibiotics, medications can contribute to Dysbiosis.

Hygiene Hypothesis: reduced early exposure to microbes may correlate to increased allergy susceptibility.

-> It's now recommended to introduce peanuts at 4-6 months to reduce risk of peanut allergy development. Previous recommendations was to wait until infant was at least 12 months.

Source:

Abbas M, Goldin J. Type I Hypersensitivity Reaction. [Updated 2025 Aug 3]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560561/

Emfietzoglou, Maria. “Atopic Triad.” Osmosis, 11 Aug. 2025, https://www.osmosis.org/answers/atopic-triad.

• Skin Prick Test (SPT): Small amounts of suspected allergens are placed on the skin and pricked. A raised, red wheal within 15–20 minutes indicates a positive reaction.

• Serum-Specific IgE (RAST or ImmunoCAP): A blood test that measures allergen-specific IgE antibodies. Useful when skin testing is contraindicated (such as with severe eczema or antihistamine use).

• Intradermal Testing: Injects allergen under the skin for more sensitive detection but carries a higher risk of systemic reaction.

• Challenge Tests: Carefully introducing allergens under medical supervision, often for food or drug allergies. Considered the “gold standard” but only done when necessary due to risk of severe reaction.

Clinical Relevance:
These tests confirm the presence of IgE-mediated hypersensitivity, guide patient education on avoidance, and direct treatment (such as immunotherapy or epinephrine use).

References:

1. Abbas, A. K., Lichtman, A. H., & Pillai, S. (2022). Basic immunology: Functions and disorders of the immune system (6th ed.). Elsevier.

2. American Academy of Allergy, Asthma & Immunology (AAAAI). (2023). Allergy testing. https://www.aaaai.org/tools-for-the-public/conditions-library/allergies/allergy-testing

3. Leung, D. Y. M., Sampson, H. A., & Geha, R. S. (2019). Allergic disorders. In J. Loscalzo, A. S. Fauci, D. L. Kasper, S. L. Hauser, D. L. Longo, & J. L. Jameson (Eds.), Harrison’s principles of internal medicine (20th ed., Vol. 2, pp. 2559–2575). McGraw Hill Education.

4. Sicherer, S. H., & Wood, R. A. (2019). Allergy testing in childhood: Using allergen-specific IgE tests. Pediatrics, 143(1), e20183604. https://doi.org/10.1542/peds.2018-3604

This case illustrates the rapid and life-threatening progression of a Type I hypersensitivity reaction, also known as anaphylaxis. The reaction is caused by IgE antibodies binding to mast cells and basophils, leading to the release of histamine and other inflammatory mediators. These substances cause vasodilation, bronchoconstriction, increased vascular permeability, and other systemic effects.

Teaching Points:

• Type I hypersensitivity reactions can occur within minutes of allergen exposure and require immediate recognition.

• Epinephrine is the first-line treatment and should be administered promptly via intramuscular injection.

• Patients should be educated on allergen avoidance, emergency action plans, and proper use of epinephrine auto-injectors.

• Observation in a medical setting is necessary following anaphylaxis due to risk of biphasic reactions.

References:

1. Simons, F. E. R., Ardusso, L. R. F., Bilo, M. B., El-Gamal, Y. M., Ledford, D. K., Ring, J., ... & Sheikh, A. (2011). World Allergy Organization anaphylaxis guidelines: 2011 update of the evidence base. International Archives of Allergy and Immunology, 154(3), 195–204. https://doi.org/10.1159/000321343

2. Sicherer, S. H., & Simons, F. E. R. (2017). Epinephrine for first-aid management of anaphylaxis. Pediatrics, 139(3), e20164006. https://doi.org/10.1542/peds.2016-4006

3. Leung, D. Y. M., Sampson, H. A., & Geha, R. S. (2019). Anaphylaxis and other immediate-type hypersensitivity reactions. In J. Loscalzo, A. S. Fauci, D. L. Kasper, S. L. Hauser, D. L. Longo, & J. L. Jameson (Eds.), Harrison’s principles of internal medicine (20th ed., Vol. 2, pp. 2566–2570). McGraw Hill Education.

4. American College of Allergy, Asthma & Immunology (ACAAI). (2022). Anaphylaxis: Symptoms, diagnosis, treatment and management. https://acaai.org/allergies/anaphylaxis

• Coughing

• Wheezing

• Shortness of breath (dyspnea)

• Chest tightness

• "Lump in throat" feeling

• Persistent clearing of throat

• Oropharyngeal swelling

References:

Abbas, M., & Goldin, J. (2025). Type I hypersensitivity reaction. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK560561/

McLendon, K., & Sternard, B. T. (2023). Anaphylaxis. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK482124/

• Hives (urticaria)

• Itching (pruritus)

• Flushing

• Rash

• Angioedema of the face, lips, and tongue.

References:

Abbas, M., & Goldin, J. (2025). Type I hypersensitivity reaction. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK560561/

McLendon, K., & Sternard, B. T. (2023). Anaphylaxis. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK482124/

• Nausea/Vomiting

• Abdominal Pain

• Cramping

• Diarrhea

References:

Abbas, M., & Goldin, J. (2025). Type I hypersensitivity reaction. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK560561/

McLendon, K., & Sternard, B. T. (2023). Anaphylaxis. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK482124/

• Hypotension

• Tachycardia

• Dizziness

References:

Abbas, M., & Goldin, J. (2025). Type I hypersensitivity reaction. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK560561/

McLendon, K., & Sternard, B. T. (2023). Anaphylaxis. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK482124/

1. Epinephrine

   • This is the most critical step that must be completed with no delay.

   • It will be administered via intramuscular (IM) injection into the patient's lateral thigh.

   • Airway constriction will reverse, the blood pressure will increase, edema will decrease, and it will stop the reaction that is occurring.

2. IV fluids & Oxygen

   • Due to the patients blood pressure dropping, IV fluids will be administered to help with hypotension.

   • Oxygen will also be given to support breathing.

3. Vasopressors, antihistamines, and bronchodilators.

   • Vasopressors are used as additional treatment to the hypotension that occurs.

   • Antihistamines are used to help relieve symptoms of urticaria (hives) and pruritus (itching).

   • Bronchodilators are used to help relieve the patient of wheezing and general difficulty with breathing.

After experiencing anaphylaxis, patients will be required to stay in the hospital for monitoring of a biphasic reaction (McLendon & Sternard, 2023).

References:

Abbas, M., & Goldin, J. (2025). Type I hypersensitivity reaction. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK560561/

McLendon, K., & Sternard, B. T. (2023). Anaphylaxis. National Library of Medicine. Retrieved October 9, 2025, from https://www.ncbi.nlm.nih.gov/books/NBK482124/

1. Respiratory System (Lungs and Airways)
   Bronchial smooth-muscle constriction, mucosal edema, and mucus hypersecretion occur.
   This leads to bronchospasm, wheezing, dyspnea, and hypoxemia.
   Airway obstruction can progress rapidly to respiratory failure.

2. Cardiovascular System (Heart and Vessels)
   Systemic vasodilation and capillary permeability cause hypotension, tachycardia, and circulatory collapse, also known as anaphylactic shock.
   Decreased venous return reduces cardiac output, worsening tissue hypoxia.

3. Skin and Mucous Membranes
   Histamine release leads to urticaria (hives), flushing, and angioedema.
   Edema of the lips, tongue, and throat can obstruct the airway.

4. Gastrointestinal System
   Smooth-muscle contraction and increased secretions cause nausea, vomiting, abdominal cramps, and diarrhea.i’m

People are more likely to develop anaphylaxis if they have prior allergen exposure or a family history of allergies, asthma, or eczema. Parenteral exposure such as injections, IV drugs, or insect stings and high-dose or repeated exposure increase the risk. Common triggers include foods, medications, insect venom, and latex. Individuals with asthma or cardiovascular disease are at higher risk for severe or fatal reactions.

• Huether, S. E., McCance, K. L., & Brashers, V. L. (2020). Understanding pathophysiology (7th ed.). Elsevier.

• National Institute of Allergy and Infectious Diseases. (2023). Anaphylaxis: Causes, symptoms, and treatment. U.S. Department of Health and Human Services. https://www.niaid.nih.gov

• Osmosis. (2022, June 17). Anaphylaxis: Causes, symptoms, and treatment [Video]. YouTube. https://www.youtube.com/watch?v=8x8kYtH3fAw