[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Immunology](https://durgical.com/en/immunology/) › [Hypersensitivity](https://durgical.com/en/immunology/#a-hypersensitivity) › Type I hypersensitivity (immediate, IgE) [Immunology](https://durgical.com/en/immunology/) · [Hypersensitivity](https://durgical.com/en/immunology/#a-hypersensitivity) # Type I hypersensitivity (immediate, IgE) - Process An allergy: the body has made IgE against something harmless, and on the next contact mast cells burst open within minutes and release histamine. Updated 2026-11-01 **See it in 3D:** [Skin](https://durgical.com/en/anatomy/skin/) · [Trachea](https://durgical.com/en/anatomy/trachea/) ## Key points - Needs prior sensitization: IgE made after a first exposure is waiting on mast cells. - Th2 cytokines: IL-4 and IL-13 for the IgE switch, IL-5 for eosinophils. - Anaphylaxis is a clinical diagnosis; intramuscular epinephrine first, without delay. - Tryptase is the mast cell enzyme that rises after anaphylaxis. - Late-phase reaction explains why asthma can worsen hours after allergen exposure. ## What it is Type I hypersensitivity is the immediate allergic reaction. A first exposure primes Th2 cells, which push B cells to switch to IgE; that IgE sits on high-affinity receptors of mast cells and basophils. On re-exposure the allergen cross-links the IgE and the cells release preformed and newly made mediators within minutes. It ranges from a runny nose to anaphylaxis, the life-threatening multisystem form. ## Step by step - First contact: dendritic cells present the allergen and naive CD4 T cells become Th2 cells. - Th2 cytokines IL-4 and IL-13 switch B cells to IgE; IL-5 recruits and activates eosinophils. - IgE binds FcεRI on mast cells and basophils: the person is now sensitized, with no symptoms yet. - Re-exposure: the allergen bridges two IgE molecules and the cell degranulates within minutes. - Early phase: histamine and tryptase cause vasodilation, leaky vessels, itching and smooth muscle spasm. - Late phase (hours later): leukotrienes, prostaglandins, cytokines and eosinophils sustain the inflammation. ## Markers - IgE - FcεRI - Histamine - Tryptase - Leukotrienes - IL-4 - IL-5 - IL-13 ## Clinical notes - Anaphylaxis: sudden skin or mucosal signs (hives, swollen lips or tongue) plus breathing trouble, low blood pressure or persistent vomiting; after a known exposure, low blood pressure alone is enough. - Epinephrine given intramuscularly is the first-line treatment; antihistamines and corticosteroids are add-ons that do not replace it and do not reverse airway or circulatory collapse. - A second wave (biphasic reaction) can come hours after the first one settles, which is why patients are observed after treatment. - Common triggers: foods (peanut, tree nuts, shellfish, milk, egg), insect venom, drugs, latex and contrast media. - Atopic diseases (allergic asthma, allergic rhinitis, atopic dermatitis) are type I and tend to cluster in the same person and family. ## In the lab - Skin prick test: a wheal and flare within minutes shows allergen-specific IgE on skin mast cells. - Serum allergen-specific IgE; total IgE and blood eosinophils are often high. - Serum tryptase drawn soon after a reaction supports anaphylaxis, but a normal value does not rule it out. ## Memory aid The mast cell is a loaded mousetrap: IgE sets it on the first visit, and the allergen springs it on the second. ## Test yourself Think of the answer, then tap the question to see it. ## Which cytokine drives the B-cell class switch to IgE? IL-4 (with IL-13), made by Th2 cells. IL-5, from the same cells, recruits and activates eosinophils. ## Which drug class is first-line in anaphylaxis, and why are antihistamines not enough? Epinephrine, given intramuscularly. It raises blood pressure and opens the airway; antihistamines only ease itch and hives and do not reverse shock or bronchospasm. ## Which blood test helps confirm that a reaction was anaphylaxis? Serum tryptase drawn soon after the event. It comes from mast cells and stays high for a few hours, but a normal result does not exclude anaphylaxis. ## Why must a person have met an allergen before to react to it in a type I reaction? Because the first contact is needed to make specific IgE and load it onto mast cells. The reaction happens on later contacts, when allergen cross-links that IgE. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/immunology/type-i-hypersensitivity/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Skin](https://durgical.com/en/anatomy/skin/) - [Trachea](https://durgical.com/en/anatomy/trachea/) ## Related - [Hypersensitivity: the four types compared](https://durgical.com/en/immunology/hypersensitivity-the-four-types-compared/) - [Mast cell and basophil](https://durgical.com/en/immunology/mast-cell-and-basophil/) - [Eosinophil](https://durgical.com/en/immunology/eosinophil/) - [IL-4, IL-5 and IL-13 (the Th2 cytokines)](https://durgical.com/en/immunology/il-4-il-5-and-il-13/) - [Latex and drug allergy](https://durgical.com/en/immunology/latex-and-drug-allergy/) - [Th1, Th2, Th17, Tfh and Treg](https://durgical.com/en/immunology/th1-th2-th17-tfh-and-treg/) ## Related conditions - [Anaphylaxis](https://durgical.com/en/conditions/anaphylaxis/) - [Asthma](https://durgical.com/en/conditions/asthma/) - [Allergic rhinitis](https://durgical.com/en/conditions/allergic-rhinitis/) [← All of immunology](https://durgical.com/en/immunology/) Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given. Sources: [Atopy](https://www.ncbi.nlm.nih.gov/books/NBK542187/) (StatPearls Publishing (NCBI Bookshelf), 2024); [Anaphylaxis](https://www.ncbi.nlm.nih.gov/books/NBK482124/) (StatPearls Publishing (NCBI Bookshelf), 2023); [Anaphylaxis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000844.htm) (MedlinePlus, U.S. National Library of Medicine, 2026). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/immunology/type-i-hypersensitivity/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/type-i-hypersensitivity/