Type I Hypersensitivity Overview

Overview

This lecture explains type I hypersensitivity (allergic) reactions, focusing on the immune mechanisms, symptoms, and common treatments.

Hypersensitivity is when the immune system causes self-damage instead of protection.
There are four types of hypersensitivity; type I is IgE-mediated and called immediate hypersensitivity.
Most allergic reactions are type I hypersensitivities.

Allergens are antigens that cause allergic reactions in genetically predisposed individuals.
The allergic response has two steps: sensitization (first exposure) and a stronger response on subsequent exposures.
Antigen-presenting cells (like dendritic cells and macrophages) present allergens to naive T-helper cells in lymph nodes.
Costimulatory molecules and cytokines (interleukin 4, 5, and 10) cause T-helper cells to become TH2 cells.
TH2 cells stimulate B cells to switch from IgM to IgE antibody production.
IgE binds to mast cells, "arming" them for a future response.

On second exposure, allergens cross-link IgE on mast cells, triggering degranulation and release of mediators.
Histamine is released, causing airway constriction, blood vessel dilation, swelling, and hives (urticaria).
Other mediators activate eosinophils and proteases, causing additional inflammation.
Early phase reactions occur within minutes; late phase reactions, 8-12 hours later, involve further immune cell recruitment.
Leukotrienes (LTB4, LTC4) contribute to prolonged inflammation and cell recruitment even after the allergen is gone.

Mild symptoms include hives, eczema, allergic rhinitis, and asthma.
Severe allergic reactions (anaphylactic shock) can be life-threatening due to airway constriction and low blood supply to organs.

Antihistamines block histamine’s effects, reducing swelling and airway constriction.
Corticosteroids suppress inflammation.
Epinephrine (EpiPen) is used for severe reactions to prevent anaphylactic shock.
Medical attention is crucial for serious allergic reactions, as symptoms may worsen.

Hypersensitivity — overactive immune response causing self-damage.
IgE (Immunoglobulin E) — antibody involved in type I hypersensitivity.
Allergen — antigen that triggers an allergic reaction.
Sensitization — initial exposure preparing immune system for later reactions.
Mast cell — immune cell that releases histamine during allergic reactions.
Histamine — mediator causing swelling, airway constriction, and hives.
Anaphylactic shock — severe allergic reaction with life-threatening airway and cardiovascular symptoms.

Review causes and mechanisms of allergic reactions.
Learn treatments for type I hypersensitivity.
Be able to identify key immune cells and mediators involved in allergic responses.