Type I Hypersensitivity: Why the Second Exposure Is the Dangerous One

The first bee sting does nothing unusual. It is the second one that can kill. This is the strange logic of type I hypersensitivity: the dangerous reaction is never to the first exposure, but to a later one. The first encounter is silent, spent quietly preparing the immune system. Only on re-exposure does the primed system fire, and it fires within minutes, sometimes fast enough to close the airway before help arrives. Understanding type I means understanding these two separate events: the silent preparation, and the explosive response. This article walks through both, and why the gap between them explains everything from a runny nose to anaphylactic shock.

Type I is the first of the four types in the Gell and Coombs classification. If you want the map of all four hypersensitivity types and how they differ, see the overview of hypersensitivity reactions. This article is about Type I: the immediate, IgE-mediated type, the one most people mean when they say "allergy," and it focuses on how the reaction works and what it causes.

The one idea that explains everything below

Type I hypersensitivity is not one event. It is two events separated by time, and almost every confusing thing about allergy comes from mixing them up.

The first event is sensitization. It is silent. The allergen is captured, shown to the immune system, and the result is IgE antibody that goes and sits on the surface of mast cells. Nothing is felt. The person walks away thinking nothing happened. This is the first bee sting.

The second event is the reaction. On a later exposure, the same allergen bridges two neighboring IgE molecules already parked on the mast cell. That bridging, called cross-linking, is the trigger. The mast cell empties its granules within minutes. This is the second bee sting.

Hold on to this split. It tells you why the first exposure is harmless, why the reaction is so fast (the antibody is already in place, so no time is lost making it), why avoiding a known allergen works, and why desensitization therapy has to be done slowly. Every section below is either describing the silent first event or the explosive second one. Keep asking which event you are reading about.

With that split in mind, here are the essentials. Type I hypersensitivity is an inappropriate, immediate immune response to common environmental substances that are otherwise harmless. Coombs and Gell placed it first in their classification of hypersensitivity reactions. IgE is the antibody that drives it.

Type I hypersensitivity reaction is called immediate hypersensitivity, as an IgE response is immediate. Similarly, it is called an anaphylactic reaction (opposite of prophylaxis or protection, i.e., harmful) or allergy. The term allergy refers to the changed reactivity of the host when encountering the antigen on a second or subsequent exposure.

Hypersensitivity type I is extremely common, where the release of pharmacological mediators, such as histamine, occurs by IgE-sensitized mast cells and produces an acute inflammatory response with symptoms like asthma or rhinitis. Manifestations can be local or systemic, leading to anaphylactic shock and even death.

Components of Hypersensitivity Type I

1. Allergen

The allergens are antigens capable of stimulating IgE production and type I hypersensitivity responses in allergic individuals on repeated exposure.

Foreign serum and egg albumin are potent antigens, whereas pollen is a weak antigen.

Table: Common allergens associated with hypersensitivity I (Source: Kuby Immunology)

2. IgE

IgE is the antibody class present in the lowest amount in normal serum, and it is the class that drives Type I hypersensitivity. Its structure and its normal role in parasite defense are covered in the article on IgE. What matters here is why it causes allergy in some people.

The answer is atopy. Atopy is an inherited tendency to make IgE against ordinary environmental substances that should be ignored, such as pollen or dust mite. In a person with atopy, a harmless antigen drives the same IgE response that would normally be reserved for a parasite. Atopic individuals can carry serum IgE levels many times higher than normal. This is why allergy runs in families, and why one person reacts to cat dander while the person next to them does not.

On the other hand, TH1 cells suppress IgE production with the help of interferon-gamma. This is the balance point: a TH2-dominant response drives IgE and allergy, while a TH1 response holds it back. IgE is also called reagin because of its role in allergic reactions. IgE is not a cytotoxic antibody. It does not lyse cells. It works by sensitizing mast cells and basophils and triggering their degranulation. The cytotoxic, cell-lysing mechanism belongs to Type II hypersensitivity, not Type I.

3. Mast cells and basophils

The basophils are granulocytes that circulate in the blood. These are recruited into tissues at the sites of inflammation.

Mast cells are found throughout connective tissue, mainly near blood, lymphatic vessels, and nerves. These are also found in the respiratory and gastrointestinal tract, skin, and mucous membranes.

Mast cells and basophils contain pharmacologically active mediators. These mediators are released from the granules on activation, manifesting a type I hypersensitivity reaction.

4. IgE binding Fc receptor

The reaginic activity of IgE depends on its ability to bind to a receptor specific to the Fc region of IgE. High-affinity and low-affinity receptors are present in receptor cells like mast cells and basophils.

5. Mediators and cytokines

Clinical manifestations of type I reaction result from mediators released during mast cells or basophil degranulation. These mediators act on local tissues and effector cells like eosinophils, neutrophils, monocytes, T lymphocytes, and platelets. If they respond to parasitic infection, the effect is beneficial. But if allergens induce the response, this results in an unnecessary increase in vascular permeability and inflammation, which is far more detrimental than helpful.

The mediators are classified as either primary or secondary. The primary mediators are pre-made before degranulation and are stored in the granules. The secondary mediators are either synthesized after target cell activation or released by the breakdown of the cell membrane during the degranulation process. The examples of primary and secondary mediators are given in the table.

Mechanism

Two steps take place in type I hypersensitivity. The first step is sensitization, and the second subsequent exposure gets serious.

Activation of B lymphocytes to produce IgE and sensitization of mast cells

This is the step where there is sensitization of the host and formation of IgE antibodies, which, once formed, attaches to the receptors on mast cells or basophil. This step is asymptomatic.

Formation of IgE

An inhaled or ingested allergen is captured by an antigen-presenting cell and carried to the lymph node. There the antigen-presenting cell shows the allergen to a helper T cell, which differentiates into a TH2 cell. The TH2 cell then releases IL-4 and IL-13, and these two signals instruct the B cell to class-switch and become a plasma cell that secretes IgE.

Read the mediator table later in this article and you will see IL-4 and IL-13 again, this time listed as mediators that increase IgE production. That is not a coincidence. The same two interleukins that start the whole process during sensitization are released again during the reaction, which is one reason an allergic person tends to stay allergic.

Binding of IgE to mast cells and basophils and sensitization

Sensitization of mast cells and basophils requires IgE. IgE has a high affinity to the Fc receptor of the mast cell and basophil cell. Thus, when these cells are coated with antibodies, they become sensitized. Locally produced IgE will first sensitize the local mast cells, and the rest of the IgE enters the circulation and binds to the Fc receptors on basophils and mast cells throughout the body.

Here is a detail that looks like trivia but explains a real clinical fact. Free IgE floating in serum has a half-life of only 2 to 3 days. But once IgE is locked onto its high-affinity receptor on a mast cell, it is protected, and the cell can stay sensitized for up to 9 to 12 weeks. This is why a person can remain allergic for months after the allergen has left their blood, and why a skin test can stay positive long after exposure. The antibody is not in the blood any more. It is sitting on the mast cell.

Mast cells release chemical mediators and adversely affect the second exposure

This is the step of reaction in response to the shocking dose. The second step includes the activation of mast cells or basophils and degranulation.

IgE cross-linkage and activation of mast cells and basophils

Months later, exposure to the same allergen cross-links the membrane-bound IgE on sensitized mast cells and basophil cells. This causes degranulation following various harmful effects.

Degranulation of the sensitized cells

The released mediators act on the surrounding tissue. They cause vasodilation and smooth muscle contraction, which produce the local or systemic signs of the reaction. Note that this is inflammation, not infection. Nothing is being infected. The damage comes from the body's own mediators, not from a microbe.

Degranulation is an active process. It requires calcium to flow into the mast cell, and it releases the primary (preformed) mediators first, followed by the secondary (newly synthesized) mediators, including cytokines and chemokines. IL-5 released here recruits and activates eosinophils, which is why eosinophils are such a feature of allergic tissue.

Table: Principal mediators involved in type I hypersensitivity (Source: Kuby Immunology)

Reactions of Immediate Hypersensitivity

Type I hypersensitivity has two presentations: an immediate reaction and a late-phase reaction.

Immediate Reaction

When a patient is exposed to an allergen, an intradermal injection of the same antigen causes redness and swelling on the injection site. This soft swelling is called a wheal, and the red rim at the margin of the wheal is a flare. This reaction appears within about 15 to 20 minutes and usually subsides within an hour. The granules are released when the allergen cross-links IgE already bound to the Fc receptors on dermal mast cells. Immediate hypersensitivity reactions include skin and mucosal allergies, food allergies, asthma, and systemic anaphylaxis.

Late phase reaction

The late phase begins 2 to 8 hours after exposure and can last 2 to 3 days. It follows the immediate wheal and flare, and sometimes appears without an obvious immediate reaction before it. It consists of an accumulation of inflammatory leukocytes, including neutrophils, eosinophils, basophils, and TH2 cells. Bronchial asthma is the classic example.

The late phase is driven by newly synthesized mediators and by the recruited inflammatory cells, and it is this sustained activity that ultimately damages tissue.

Clinical manifestations

The type 1 reaction can range from the life-threatening anaphylactic reaction to milder forms associated with common allergies.

Immediate hypersensitivity reactions are seen in the URT (upper respiratory tract), GIT (gastrointestinal tract), and skin. Most allergic responses occur on the mucus membrane surface when an allergen enters the body by inhalation or ingestion. Vascular and smooth muscle reaction develops after repeated exposure to the allergen (the immediate response).

Diseases associated with hypersensitivity I can be described in two types: localized and systemic.

1. Localized Reaction

• Skin allergies, Hay fever, allergic rhinitis, asthma, food allergy
• Rashes or blisters in the skin, pruritus (hives, atopic dermatitis, eczema)
• Increased eye and nasal secretions, itching, sneezing (allergic rhinitis/hay fever, allergic conjunctivitis)
• Oropharyngeal mucosal edema (food allergies)
• Gastrointestinal abnormalities, like abdominal pain, diarrhea, vomiting (food allergies)
• Bronchospasm, wheezing (bronchial asthma)
• Urticaria and eczema

2. Systemic anaphylaxis

• Anaphylaxis is a severe, life-threatening systemic hypersensitivity reaction occurring within minutes of exposure to an allergen.
• Large quantities of inflammatory mediators are released due to rapid systemic vasodilation, vascular permeability, hypotension, and extensive tissue edema. Fluids may be in the lungs and constrict the airways. Patients can undergo shortness of breath, lethal suffocation, cardiovascular collapse, and loss of consciousness.

Lab Diagnosis

Skin testing

Hypersensitivity type I can commonly be identified and assessed by skin testing. A small amount of potential allergen is introduced at a specific skin site. Wheal and flare are seen within 30 minutes, which confirms the type I reaction.

Serum testing

Serum total IgE and allergen-specific IgE are measured by immunoassay. Older methods used radioimmunoassay (the original RAST), but current laboratories use enzyme or fluorescence immunoassay platforms such as ImmunoCAP.

Control

Type I hypersensitivity can be controlled by applying the following measures:

• Contact with known allergens should be avoided. Removing house pets, dust, and offending foods should be done as far as possible.
• In severe cases, hyposensitization or desensitization can help patients, i.e., immunotherapy with repeated injections of increasing doses of allergens, to reduce the severity or to eliminate them.
• The use of humanized monoclonal anti-IgE can also be helpful.
• Drug therapies, like, antihistamines, are most useful in allergic rhinitis. Glucocorticoids are also used.
• Anaphylaxis is a medical emergency requiring immediate airway access with the administration of epinephrine and fluid resuscitation.

How to remember Type I

Two bee stings. The whole topic collapses into one image. First sting: silent, makes the IgE, arms the mast cells. Second sting: the allergen bridges the IgE, the mast cell fires in minutes. If you can hold "the first one arms, the second one fires," you can rebuild the entire mechanism from memory.

Primary is preformed, secondary is synthesized. Both start with the same letter as their timing. Primary mediators (histamine, heparin, tryptase) are Preformed and sit ready in the granule. Secondary mediators (leukotrienes, prostaglandins, PAF, cytokines) are Synthesized fresh after the trigger. Timing tells you the type.

"Anaphylaxis" is the opposite of "prophylaxis." Prophylaxis means protection. Ana- reverses it. The word itself tells you this is protection turned harmful.

IgE, the number "one." Type ONE is IgE, and IgE is the antibody present in the lowest amount, number one from the bottom. Two ones link the type to its antibody.

Key exam facts in one table

Where students get confused

"The reaction is to the first exposure." No. The first exposure causes no symptoms. It only builds and positions the IgE. Symptoms need a second (or later) exposure to the same allergen. The dangerous event is never the first one.

"IgE is a cytotoxic antibody." No. IgE does not kill or lyse cells. It sensitizes mast cells and basophils and triggers their degranulation. The cell-killing, cytotoxic antibody mechanism is Type II hypersensitivity. Mixing these up is one of the most common exam errors.

"Immediate and late-phase are two different diseases." They are two stages of the same reaction in the same person. The immediate phase (minutes) comes from preformed mediators like histamine. The late phase (hours) comes from newly recruited inflammatory cells and newly synthesized mediators. One allergen exposure can produce both.

"Histamine causes the whole thing." Histamine causes the fast, early signs. But the sustained wheezing and tissue damage of asthma owe more to leukotrienes and to the cells recruited in the late phase. This is why antihistamines help hay fever but do not control asthma on their own.

"A high IgE means allergy." Not necessarily. Serum IgE also rises in parasitic infection, which is IgE's normal job. A raised IgE has to be read against the clinical picture, not taken as proof of allergy by itself.

"Anaphylaxis is a severe local allergy." Anaphylaxis is systemic by definition. Mediators are released body-wide, causing widespread vasodilation, a drop in blood pressure, and airway compromise. A severe local reaction is not anaphylaxis.

References

1. Punt J, Stranford SA, Jones PP, Owen JA. Kuby Immunology. 8th ed. New York: W. H. Freeman; 2019.
2. Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Philadelphia: Elsevier; 2022.
3. Delves PJ, Martin SJ, Burton DR, Roitt IM. Roitt's Essential Immunology. 13th ed. Chichester: Wiley-Blackwell; 2017.
4. Dullaers M, De Bruyne R, Ramadani F, et al. The who, where, and when of IgE in allergic airway disease. J Allergy Clin Immunol. 2012;129(3):635-645.