Immediate hypersensitivity, known as a Type I hypersensitivity reaction, is a rapid and heightened immune response to substances that are harmless. These substances, called allergens, can include things like pollen, certain foods, or insect venom. The defining characteristic of this reaction is its speed; symptoms manifest quickly, often within minutes to a couple of hours after someone is exposed to their specific trigger. The reaction can range from mild irritation to a severe, body-wide event.
The Sensitization Phase
An allergic reaction begins with a “sensitization phase” during the first-ever contact with a potential allergen. During this initial exposure, no outward symptoms appear because the immune system is being silently primed. When a substance like pollen or a food protein enters the body, antigen-presenting cells identify it as a threat. This initiates a response within the immune system.
These signals instruct a specific type of white blood cell, the plasma cell, to begin producing antibodies called Immunoglobulin E (IgE). Unlike antibodies made to fight infections, this IgE is tailored specifically to the allergen. These allergen-specific IgE antibodies then enter the bloodstream and circulate throughout the body.
The newly created IgE antibodies do not remain free-floating for long. Their primary destination is the surface of two types of immune cells: mast cells and basophils. Mast cells are located in tissues that are common entry points for allergens, such as the skin, the lining of the nose and airways, and the digestive tract. Basophils circulate in the blood. The IgE antibodies attach to receptors on these cells, turning them into “sensitized” cells, armed and waiting for a future encounter with that specific allergen.
The Elicitation Phase
Once an individual is sensitized, any subsequent exposure to the same allergen will trigger the elicitation phase. This phase is much more rapid than sensitization. When the allergen re-enters the body, it directly interacts with the IgE antibodies already anchored to the surfaces of mast cells and basophils.
This binding event is the trigger for the reaction. The allergen connects with and cross-links the IgE antibodies on the cell surface, sending an activation signal into the mast cell or basophil. This signal causes the cells to undergo a process called degranulation, a release of chemical mediators that have been stored inside them. This release is immediate and floods the surrounding tissues with these compounds.
Histamine is the most well-known chemical mediator and is responsible for many of the classic, immediate signs of an allergy. Alongside histamine, the activated cells also release other inflammatory substances, such as leukotrienes and prostaglandins. These chemicals act on local blood vessels, smooth muscles, and nerve endings to produce the physical symptoms associated with the reaction.
Clinical Manifestations and Symptoms
The chemical mediators released during the elicitation phase produce a wide spectrum of physical symptoms. These clinical manifestations can be localized to a specific area of the body or can affect the entire system, depending on the severity of the reaction and the route of allergen exposure.
Many reactions are localized. For example, histamine’s effect on blood vessels in the skin can cause them to leak fluid, leading to the raised, itchy welts known as hives (urticaria) and deeper swelling called angioedema. In the respiratory tract, these mediators can cause the nasal passages to swell and produce excess mucus, resulting in the symptoms of allergic rhinitis or hay fever. When the lower airways are affected, the smooth muscles can constrict, leading to the wheezing and shortness of breath of allergic asthma.
In some cases, the reaction becomes a systemic, whole-body event known as anaphylaxis. This is the most severe form of immediate hypersensitivity, where large amounts of mediators are released into the bloodstream, affecting multiple organ systems. This can cause severe swelling of the throat and tongue, obstructing the airway. It can also lead to a sudden drop in blood pressure, known as anaphylactic shock, causing dizziness, a rapid heart rate, and potential loss of consciousness. Anaphylaxis is a medical emergency.
Common Triggers
Many substances, or allergens, can provoke an immediate hypersensitivity reaction in sensitized individuals. These triggers are grouped by how they enter the body. The specific substance that causes a reaction in one person may be completely harmless to another.
Inhaled allergens are among the most common triggers for respiratory symptoms. This category includes airborne particles such as pollen from trees, grasses, and weeds, which cause seasonal allergies. Other prevalent inhaled allergens are perennial, including dust mites, mold spores, and dander from animals like cats and dogs.
Ingested allergens are found in foods and are a frequent cause of reactions ranging from hives to life-threatening anaphylaxis. While any food can be an allergen, a few are responsible for the majority of food allergies:
- Peanuts
- Tree nuts (like walnuts and almonds)
- Milk
- Eggs
- Soy
- Wheat
- Fish
- Shellfish
Allergens can also be injected directly into the body. Insect stings from bees, wasps, hornets, and fire ants are a primary example, capable of causing severe systemic reactions. Certain medications, such as penicillin, can also trigger immediate hypersensitivity when administered via injection or taken orally. Lastly, some allergens, such as latex, can cause reactions through direct contact with the skin, though latex particles can also be inhaled.