Immediate Hypersensitivity Reactions



The immune system is an integral part of human protection against disease, but the normally protective immune mechanisms can sometimes cause detrimental reactions in the host. Such reactions are known as hypersensitivity reactions, and the study of these is termed immunopathology. The traditional classification for hypersensitivity reactions is that of Gell and Coombs and is currently the most commonly known classification system.[1] It divides the hypersensitivity reactions into the following 4 types:

Some authors believe this classification system may be too general and favor a more recent classification system proposed by Sell et al.[3] This system divides immunopathologic responses into the following 7 categories:

This system accounts for the fact that multiple components of the immune system can be involved in various types of hypersensitivity reactions. For example, T cells play an important role in the pathophysiology of allergic reactions (see Pathophysiology). In addition, the term immediate hypersensitivity is somewhat of a misnomer because it does not account for the late-phase reaction or for the chronic allergic inflammation that often occurs with these types of reactions.

Allergic reactions manifest clinically as anaphylaxis, allergic asthma, urticaria, angioedema, allergic rhinitis, some types of drug reactions, and atopic dermatitis. These reactions tend to be mediated by IgE, which differentiates them from pseudoallergic (formerly called anaphylactoid) reactions that involve IgE-independent mast cell and basophil degranulation. Such reactions can be caused by iodinated radiocontrast dye, opiates, or vancomycin and appear similar clinically by resulting in urticaria or anaphylaxis.[4, 5]

Patients prone to IgE-mediated allergic reactions are said to be atopic. Atopy is the genetic predisposition to make IgE antibodies in response to allergen exposure.[6]

The focus of this article is allergic reactions in general. Although some of the clinical manifestations listed previously are briefly mentioned, refer to the articles on these topics for more detail. For example, see Allergic and Environmental Asthma; Anaphylaxis; Food Allergies; Rhinitis, Allergic; and Urticaria.


Immediate hypersensitivity reactions are mediated by IgE, but T and B cells play important roles in the development of these antibodies. T helper (TH) cells, which are CD4+, have been divided into 2 broad classes based on the cytokines they produce: TH1 and TH2.[7, 8] Regulatory T cells (Tregs) are CD4+CD25+ and may also play a role.[9]

TH1 cells produce interferon gamma, interleukin (IL)–2, and tumor necrosis factor-beta and promote a cell-mediated immune response (eg, delayed hypersensitivity reaction). TH2 cells, on the other hand, produce IL-4 and IL-13, which then act on B cells to promote the production of antigen-specific IgE. Therefore, TH2 cells play an important role in the development of immediate hypersensitivity reactions, and patients who are atopic are thought to have a higher TH2-to-TH1 cell ratio. Interestingly, the cytokines produced by TH1 cells (specifically interferon gamma) seem to diminish the production of TH2 cells.[10, 7, 8] Current evidence suggests that Tregs may also actively inhibit TH2 responses to allergens.[9]

The allergic reaction first requires sensitization to a specific allergen and occurs in genetically predisposed individuals. The allergen is either inhaled or ingested and is then processed by the dendritic cell, an antigen-presenting cell.[11] The antigen-presenting cells then migrate to lymph nodes, where they prime naive TH cells (TH0 cells) that bear receptors for the specific antigen.

TH0 cells are undifferentiated CD4 cells that release both TH1 and TH2 cytokines and can develop into either cell type. In the case of allergen sensitization, the TH0 cells are thought to be exposed to IL-4 (from as yet unidentified sources, but including germinal-center B cells) and possibly to histamine-primed dendritic cells, both of which cause them to develop into TH2 cells. These primed TH2 cells then release more IL-4 and IL-13. IL-4 and IL-13 then act on B cells to promote production of antigen-specific IgE antibodies.

For this to occur, B cells must also bind to the allergen via allergen-specific receptors. They then internalize and process the antigen and present peptides from it, bound to the major histocompatibility class II molecules found on B-cell surfaces, to the antigen receptors on TH2 cells. The B cell must also bind to the TH2 cell and does so by binding the CD40 expressed on its surface to the CD40 ligand on the surface of the TH2 cell. IL-4 and IL-13 released by the TH2 cells can then act on the B cell to promote class switching from immunoglobulin M production to antigen-specific IgE production (see image below).

View Image

Immediate hypersensitivity reactions. Sensitization phase of an immunoglobulin E–mediated allergic reaction.

The antigen-specific IgE antibodies can then bind to high-affinity receptors located on the surfaces of mast cells and basophils. Reexposure to the antigen can then result in the antigen binding to and cross-linking the bound IgE antibodies on the mast cells and basophils. This causes the release and formation of chemical mediators from these cells. These mediators include preformed mediators, newly synthesized mediators, and cytokines. The major mediators and their functions are described as follows:[7, 8]

Preformed mediators

Newly formed mediators


The actions of the above mediators can cause variable clinical responses depending on which organ systems are affected, as follows:

Allergic reactions can occur as immediate reactions, late-phase reactions, or chronic allergic inflammation. Immediate or acute-phase reactions occur within seconds to minutes after allergen exposure. Some of the mediators released by mast cells and basophils cause eosinophil and neutrophil chemotaxis. Attracted eosinophils and resident lymphocytes are activated by mast cell mediators.

These and other cells (eg, monocytes, T cells) are believed to cause the late-phase reactions that can occur hours after antigen exposure and after the signs or symptoms of the acute-phase reaction have resolved. The signs and symptoms of the late-phase reaction can include redness and swelling of the skin, nasal discharge, airway narrowing, sneezing, coughing, and wheezing. These effects can last a few hours and usually resolve within 24-48 hours.

Finally, continuous or repeated exposure to an allergen (eg, a cat-owning patient who is allergic to cats) can result in chronic allergic inflammation. Tissue from sites of chronic allergic inflammation contains eosinophils and T cells (particularly TH2 cells). Eosinophils can release many mediators (eg, major basic protein), which can cause tissue damage and thus increase inflammation. This can result in structural and functional changes to the affected tissue. Furthermore, a repeated allergen challenge can result in increased levels of antigen-specific IgE, which ultimately can cause further release of IL-4 and IL-13, thus increasing the propensity for TH2 cell/IgE–mediated responses.[8]



United States







History findings vary depending on which organ systems are affected.


Physical examination findings vary with the organ system involved.


Atopy is defined as the genetic predisposition to form IgE antibodies in response to exposure to allergens. Therefore, a genetic predisposition exists for the development of atopic diseases. Mutations of specific alleles on the long arm of chromosome 5 have been associated with higher levels of IL-4 and IgE and are known as IL-4 promoter polymorphisms.[31] Impaired function of Treg cells may also contribute to the development of atopic diseases.[32]

Environmental issues also play an important role, although the role that exposure at an early age to certain antigens might play in either the progression to or the protection from the development of an allergic response remains unclear. Some studies have shown that children in day care and those with older siblings may be less likely to develop allergic disease. The environment certainly can help determine the allergens to which the patient will be exposed. For example, children in inner cities are more likely to be sensitized to cockroaches than are children in suburban or rural areas. Similarly, dust mites, a potent allergen, are primarily found in humid climates, and those who have never been exposed to such a climate are less likely to be allergic to mites.

Laboratory Studies

Some laboratory tests may be helpful in determining whether a reaction is truly allergic in nature.

Obtaining a serum tryptase level soon after the onset of symptoms can be helpful in differentiating anaphylaxis from other forms of shock and from other symptom complexes that may be confused with anaphylaxis. The tryptase level can be elevated, which is indicative of mast cell degranulation. False-negative results can occur. Ideally, the tryptase level should be drawn within 4 hours after the event, but it can be drawn up to 15 hours later. Measurement of urinary histamine may also be useful.

An elevated eosinophil count may be observed in patients with atopic disease.

IgE levels may be elevated in patients who are atopic, but the level does not necessarily correlate with clinical symptoms.

The radioallergosorbent test (RAST) and other in vitro IgE assays measure antigen-specific IgE and can be useful in identifying which allergens are causing symptoms for the patient. More sensitive tests have been available in recent years and have a greater positive predictive value for foods. These tests can sometimes detect clinically irrelevant allergens, however, creating false-positive results to some foods. Molecular diagnostic tests are now available that can detect allergenicity to specific food protein molecules known to trigger IgE-mediated reactions.[33]

Other Tests

Medical Care

Treatment may vary depending on the type of allergic reaction. Some general observations are made below, but refer to articles on the specific topics for more details about treatment (eg, Anaphylaxis; Rhinitis, Allergic; Allergic and Environmental Asthma; Urticaria).



Medication Summary

Medical therapy varies somewhat depending on which type of allergic reaction is being treated. Some of the drugs and their categories are listed here, but refer to the articles on the specific allergic reaction for more detail.

Epinephrine (Adrenalin, Bronitin, EpiPen, Twinject)

Clinical Context:  Should be administered immediately for anaphylaxis/anaphylactic shock. Multiple preparations allow for delivery SC, IM, IV, or ET. Doses can be repeated q5min prn to maintain blood pressure (and as heart rate allows).

Class Summary

First-line choice to reverse effects of systemic vasodilation and increased vasopermeability observed with anaphylaxis. Although not the first choice for bronchoconstriction, epinephrine can also relieve some symptoms of bronchospasm and rhinitis. In the past, protocols called for subcutaneous or intravenous administration of epinephrine. However, studies have shown that intramuscular epinephrine leads to higher plasma levels than subcutaneous delivery. Intramuscular administration is now preferred over subcutaneous administration.[43, 44]

Predosed autoinjectable epinephrine is available in at least 2 forms, which include EpiPen and Twinject. Two doses of each are available (0.3 mg for EpiPen or Twinject 0.3 and 0.15 mg for EpiPen Jr. or Twinject 0.15). One Twinject pen actually has 2 doses of epinephrine available, which can be administered separately, and also has directions printed on a wraparound label on the pen that can be referred to at the time of use. The EpiPen has also been redesigned for easier use and has instructions on the pen. EpiPen Two-packs contain 2 pens and, therefore, 2 doses. Twinject Two-packs contain 2 pens for a total of 4 doses.

Albuterol (ProAir HFA, Ventolin HFA, Proventil HFA)

Clinical Context:  Sympathomimetic that stimulates beta-2 receptors, leading to bronchodilation. Used for bronchospasm refractory to epinephrine with anaphylaxis. First-line choice for acute bronchospasm associated with asthma.

Salmeterol/fluticasone (Advair); formoterol/budesonide (Symbicort); formoterol/mometasone (Dulera)

Clinical Context:  Fluticasone, budesonide, and mometasone are steroids. They inhibit bronchoconstriction mechanisms, may decrease number and activity of inflammatory cells, in turn decreasing airway hyper-responsiveness. Also may have vasoconstrictive activity.

Salmeterol and formoterol relax the smooth muscles of the bronchioles in conditions associated with bronchitis, emphysema, asthma, or bronchiectasis, and can relieve bronchospasm. Effect may also facilitate expectoration. Advair is available both as an HFA-propelled MDI and as a dry powder inhaler. Symbicort and Dulera are only available as an MDI.

Adverse effects are more likely to occur when administered at high or more frequent doses than recommended.

Class Summary

Inhaled bronchodilators are beta-agonists that come in short- and long-acting forms. Short-acting bronchodilators are used to treat acute bronchospasm. Can also be used prophylactically. For example, a patient with a history of asthma exacerbation in the presence of cats can use a short-acting bronchodilator before exposure to cats. Long-acting bronchodilators (eg, salmeterol) can be used twice daily and to help maintain bronchodilation over 12 h. In asthma patients, they should only be used in conjunction with inhaled glucocorticoids.

Pirbuterol and formoterol have both short- and long-acting activity. Onset of action is approximately 15 min, but effects last up to 12 h. Of note, when long-acting beta agonists are used alone, concern exists of increased mortality in asthma patients. These medications should be combined with an inhaled corticosteroid and should be reserved for patients with more frequent or moderate to severe symptoms or lung function. Finally, levalbuterol is the R-enantiomer of albuterol and is available in nebulizer and metered dose inhaler (MDI) forms. Advantage of levalbuterol is that it is less likely to cause paradoxical bronchospasm than racemic albuterol.

Previously, MDIs were made using chlorofluorocarbons (CFCs) as the propellant. However, the use of CFCs has been phased out because of environmental concerns. For this reason, companies are now making MDIs with hydrofluoroalkane-134a (HFA), which is not damaging to the ozone layer. CFC inhalers are no longer available in the United States.

Prednisone (Sterapred)

Clinical Context:  Believed to ameliorate delayed effects of anaphylactic reactions and may limit biphasic anaphylaxis. Doses below are general guidelines for usage; dosing is highly individualized.

Class Summary

Immunosuppressing agents and, thus, can decrease inflammation. Have particular efficacy in skin eruptions and bronchospasm. Role in anaphylactic shock is limited, although believed to help prevent delayed type of anaphylaxis.

Several different formulations are available; only one is listed. Others include methylprednisolone, dexamethasone, prednisolone (often used in children), and hydrocortisone. Depending on type of corticosteroid, oral, intravenous, and topical forms may be available. In more severe cases of anaphylaxis and asthma, intravenous forms of corticosteroids can be used initially. These can later be switched to oral forms as doses are tapered.

Inhaled corticosteroids are another form of corticosteroids and are key in controlling inflammation of bronchial airways and nasal mucosa. Similarly, topical corticosteroids are useful in treating atopic dermatitis.

Diphenhydramine (Benadryl, Dihydrex injection, Belix)

Clinical Context:  Most widely available antihistamine (available OTC). Sedating antihistamines may be necessary to control more severe allergic reactions because they are very potent. Dosing interval of diphenhydramine is 4-6 h. Nonsedating antihistamines are all now available in a 24-h formulation but can only be administered PO.

Azelastine (Astelin)

Clinical Context:  An effective antihistamine delivered via the intranasal route. Mechanism is similar to oral antihistamines. Systemic absorption occurs and may cause sedation, headache, and nasal burning.

Forms complex with histamine for H1-receptor sites in blood vessels, GI tract, and respiratory tract.

Use prn or qd. Use alone or in combination with other medications. Unlike oral antihistamines, has some effect on nasal congestion. Helpful for vasomotor rhinitis. Some patients experience a bitter taste. Systemic absorption may occur, resulting in sedation (reported in approximately 11% of patients).

Cetirizine (Zyrtec)

Clinical Context:  Selectively inhibits histamine H1 receptor sites in blood vessels, GI tract, and respiratory tract, which in turn inhibits physiologic effects that histamine normally induces at H1 receptor sites. Once-daily dosing is convenient. Bedtime dosing may be useful if sedation is a problem.

Class Summary

Type 1 histamine-receptor blockers act to block action of histamine on H1 receptor after its release from mast cells and basophils. Most effective when used prophylactically. Sedating first-generation and nonsedating second-generation H1 antihistamines are available. Typically, sedating antihistamines have more adverse anticholinergic effects. Sedating antihistamines include diphenhydramine, hydroxyzine, cyproheptadine, chlorpheniramine, and brompheniramine. Nonsedating antihistamines include cetirizine (cause drowsiness in 15% people), fexofenadine, loratadine, and desloratadine. Desloratadine and fexofenadine may also help decrease nasal congestion. Liquid forms are more rapidly absorbed orally and should be used for immediate treatment of an allergic reaction if intravenous access is not available.

Ranitidine (Zantac)

Clinical Context:  Multiple formulations are available. Cimetidine was first to be widely used but tends to have more drug interactions than other H2-receptor blockers. If no response to H1-receptor antagonist alone, coadministration with an H2-receptor antagonist can help relieve symptoms of itching and flushing in anaphylaxis, pruritus, and urticaria. Cimetidine plus an H1 blocker blocks cardiovascular effects of histamine.

Class Summary

Can be administered in addition to H1-receptor blockers for additional control of urticaria and angioedema. Examples include ranitidine, famotidine, and cimetidine. Cimetidine has been studied more extensively for this indication than other members of this class.

Montelukast (Singulair)

Clinical Context:  Leukotriene inhibitors can be a helpful addition to asthma and allergic rhinitis not well controlled with H1-receptor blockers and inhaled corticosteroids.

Class Summary

Leukotrienes are synthesized by degranulated mast cells and basophils and likely contribute significantly to symptoms of allergic reactions. Three leukotriene inhibitors are now available in the United States. Montelukast and zafirlukast act as leukotriene D4-receptor blockers, whereas zileuton acts to inhibit production of leukotrienes B4, C4, D4, and E4. Disadvantages of the latter medication are its qid dosing and the need to monitor liver enzymes. However, a sustained release formulation is now available, permitting q12 h dosing; zileuton’s ability to inhibit synthesis of leukotrienes C4 and B4 as well as D4 may lead to increased use of this leukotriene modulator for certain indications.

Tacrolimus ointment (Protopic)

Clinical Context:  Reduces itching and inflammation by suppressing release of cytokines from T cells. Can be used in patients as young as 2 y. More expensive than topical corticosteroids.

Class Summary

Tacrolimus is a calcineurin inhibitor initially used in oral form as an immunosuppressant for transplantation patients. It has since been developed in topical form (Protopic) and can be used to treat atopic dermatitis that does not respond well to topical corticosteroids. A similar topical agent, pimecrolimus (Elidel), became available in the past few years and is indicated for mild atopic dermatitis. Systemic calcineurin inhibitors have been shown to cause immunosuppression and certain malignancies such as lymphoma. In January 2006, the FDA issued a black box warning for topical tacrolimus and pimecrolimus for these reasons.[41] To date, studies have not shown significant systemic absorption, systemic immunosuppression, or increased risk of malignancy with the topical formulations. Trials are currently underway to assess possible benefit of inhaled tacrolimus for asthma.

Omalizumab (Xolair)

Clinical Context:  Binds to IgE and thereby prevents IgE from binding to mast cells and basophils.

Class Summary

Omalizumab (Xolair) is a monoclonal anti-IgE antibody indicated for refractory asthma. Has been shown to greatly improve severity of asthma in patients and can be used to help patients dependent on oral steroids to be weaned from steroids. Omalizumab has also been shown to decrease allergic response to peanuts in patients with severe peanut allergy.[45] This could be helpful in preventing anaphylaxis from accidental peanut exposure in patients who normally would not tolerate even the slightest exposure to peanut allergen, but it only has FDA approval for asthma at this time. Patients should undergo a full allergy evaluation prior to starting omalizumab, if needed, because it interferes with prick skin test and RAST results.



Miriam K Anand, MD, FAAAAI, FACAAI, Consulting Staff, Department of Allergy/Immunology, Allergy Associates and Lab, Ltd; Clinical Assistant Professor, Midwestern School of Osteopathic Medicine

Disclosure: TEVA pharmaceuticals Honoraria Speaking and teaching


John M Routes, MD, Professor of Pediatrics, Medicine, Microbiology and Molecular Genetics, Chief, Section of Allergy and Clinical Immunology, Department of Pediatrics, Medical College of Wisconsin

Disclosure: Nothing to disclose.

Specialty Editors

Francisco Talavera, PharmD, PhD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Michael R Simon, MD, MA, Clinical Professor Emeritus, Departments of Internal Medicine and Pediatrics, Wayne State University School of Medicine; Professor, Department of Internal Medicine, Oakland University William Beaumont University School of Medicine; Adjunct Staff, Division of Allergy and Immunology, Department of Internal Medicine, William Beaumont Hospital

Disclosure: Secretory IgA, Inc. Ownership interest Management position; siRNAx, Inc. Ownership interest Management position

Timothy D Rice, MD, Associate Professor, Departments of Internal Medicine and Pediatrics and Adolescent Medicine, St Louis University School of Medicine

Disclosure: Nothing to disclose.

Chief Editor

Michael A Kaliner, MD, Clinical Professor of Medicine, George Washington University School of Medicine; Chief, Section of Allergy and Immunology, Washington Hospital Center; Medical Director, Institute for Asthma and Allergy

Disclosure: Teva Honoraria Speaking and teaching; Meda Honoraria Speaking and teaching; genentech Honoraria Speaking and teaching; sunovian Consulting fee Consulting

Additional Contributors

The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous author John M Routes, MD, to the development and writing of this article.


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Immediate hypersensitivity reactions. Sensitization phase of an immunoglobulin E–mediated allergic reaction.

Immediate hypersensitivity reactions. Sensitization phase of an immunoglobulin E–mediated allergic reaction.