What is type I hypersensitivity? Type I hypersensitivity, or sometimes IgE-mediated hypersensitivity or immediate hypersensitivity, is a type of immune reaction in which tissue is damaged due to IgE antibody.

Most allergies are caused by type I hypersensitivity reactions. This video covers the pathophysiology, complications, signs and symptoms, and treatment of type I hypersensitivity reactions.

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Hypersensitivity is a condition in which someone’s immune system reacts in a way that damages their body instead of healing or protecting it. There are four types of hypersensitive reactions.

Type one reactions involve an antibody called immunoglobulin E, also referred to as IgE. Type one reactions are also called IgE mediated hypersensitivities. Type one reactions occur within minutes, so they are also sometimes referred to as immediate hypersensitivity.

Most allergic reactions are type one hypersensitivity reactions. Allergies are reactions to antigens from outside the body that other people don’t commonly react to. Common allergens include food, bee stings, latex, and soaps. Sensitization occurs during the first exposure to an allergen. Reactions after subsequent exposures will be much more serious. Allergies may be caused by genetics that cause hypersensitive T helper cells.

During an allergic reaction, T helper cells bind to the triggering molecule and form an allergen. The allergen is then picked up by other immune cells like dendric cells and macrophages. The immune cells carry the allergen to the lymph nodes and present it to T helper cells. If the person is allergic to the allergen, the presenting cells express co-stimulatory molecules to help with immune response.

Before the T helper cells are presented with the allergen, they are referred to as naïve T helper cells. After the allergen is presented, the T helper cells become type two helper cells, or TH2. The change is triggered by the co-stimulatory molecules, small proteins, and interleukins. The TH2 cells release more interleukins, which trigger B cells to stop making IgM antibodies and start making IgE antibodies specific to the allergen. Interleukins released by TH2 cells also trigger production of eosinophils, which in turn release chemicals toxic to the allergen as well as nearby cells. The IgE antibodies attach themselves to receptors on mast cells, priming them for attack and completing the sensitization phase.

Upon second exposure to the allergen, the mast cells with attached IgE antibodies bind the allergen; once two or more have bound the allergen, the mast cells receive a signal and release inflammatory mediators, which trigger the effects of an allergic reaction.

Histamine is one of the major inflammatory mediators released during an allergic reaction. Histamine causes the smooth muscles of the bronchi to contract, which causes the airway to swell or close and makes breathing difficult. Histamine also dilates the blood vessels and increases permeability of the vessel walls, which causes swelling and hives.

Mast cells release other inflammatory mediators that trigger early phase reactions within minutes of second exposure to the allergen. During early phase reactions, eosinophils and proteases degrade large proteins into small peptides.

Late phase reactions may also occur between eight and twelve hours after exposure. Late phase reactions are triggered by the cytokines and inflammatory mediators produced during early phase reactions. Late phase reactions involve the same immune cells as early phase reactions, with the addition of small fatty acid molecules called leukotrienes. Like histamines, leukotrienes cause smooth muscle contractions in the respiratory system. They also attract immune cells to the location long after the allergen has been removed.

Symptoms of mild allergic reaction include eczema, asthma, and hives. In more serious reactions, anaphylactic shock may occur if vital organs aren’t supplied with sufficient amounts of oxygen.

Antihistamines may be used to block the effects of histamine during an allergic reaction. Corticosteroids can be used to reduce inflammation. Epinephrine is used to constrict the blood vessels and prevent anaphylactic shock.  Medical attention should be sought in the case of type one hypersensitivity reactions.