Type I Hypersensitivity (HS)
Type I Hypersensitivity (or immediate hypersensitivity) is an allergic reaction provoked by re-exposure to a specific antigen called an allergen. Exposure may be by ingestion, inhalation, injection, or direct contact.
B-cells are stimulated by TH2 cells to produce IgE specific to an antigen (Fig 1). The difference between a normal immune response to an infection and a type 1 HS response is that in type 1 HS, the antibody is IgE instead of IgA, IgG, or IgM.
During sensitization, IgE binds to antibody receptors on the surface of tissue mast cells and circulating basophils. Mast cells and basophils coated by IgE are thus "sensitized". Later exposure to the same allergen results in the allergen cross-linking the bound IgE on sensitized cells which results in mast cell degranulation.
Mast cell degranulation causes immediate and explosive release of various immune mediators from storage granules. Some of these mediators include histamine, leukotrienes and prostaglandin, which act on proteins located on surrounding tissues. The resulting effect is vasodilation and smooth-muscle contraction.
Figure 1: Major Clinical Features resulting from mast cell degranulation
Type 1 HS may be either immediate or late (Fig 2). The immediate HS reaction occurs minutes after exposure and includes release of vasoactive amines and lipid mediators, whereas the late reaction occurs 2–4 hours after exposure and includes the release of cytokines.
Figure 2: Stages of Mast Cell release of inflammatory mediators
Many immune mediators are released by mast cells during a type 1 HS resulting in multiple actions. Histamine, Leukotriene and Prostaglandin causes vasodilation with increased vascular permeability and smooth muscle spasm. The cytokines and chemokines causes Leukocyte extravasation ie white blood cells leave the blood vessels resulting in inflammation.The reaction may be either local or systemic. Symptoms vary from mild irritation to anaphylactic shock.
Examples of clinical conditions include allergic asthma, conjunctivitis, rhinitis, urticaria, anaphylaxis, angioedema, drugs such as Penicillins and Cephalosporins, foods such as peanuts.
Immediate treatment is CPR and Epipen®. Thereafter, the patient should be admitted for general support and specific drugs such adrenaline (epinephrine), antihistamines and corticosteroids. Anaphylaxis may require ICU admission and critical care.
Dr Moodley will be speaking more around hypersensitivity on 6 May 2018. Click here to book a place https://www.belmatt.co.uk/paediatric-allergies