Overview
This lecture explains hypersensitive (allergic) reactions, focusing on type 1 hypersensitivity, its mechanisms, and clinical outcomes in various body systems.
Hypersensitive Reactions Overview
- Hypersensitive reactions are excessive immune responses to harmless substances, causing inflammation and tissue damage.
- There are four basic types of hypersensitive reactions.
Type 1 Hypersensitivity (Allergic Reactions)
- Type 1 reactions involve allergens, IgE antibodies, and mast cells.
- Common triggers include medicines (e.g., penicillin), foods (e.g., nuts), chemicals, and pollen.
- Genetic susceptibility increases risk of developing allergic reactions.
- Affected body areas include the gastrointestinal tract, airway, and skin.
Mechanism of Type 1 Reaction
- On first exposure to an allergen (e.g., pollen), antigen-presenting cells (macrophages/dendritic cells) engulf and present the allergen to T helper cells in local lymph nodes.
- T helper cells activate specific B cells, which then become plasma B cells producing IgE antibodies against the allergen.
- IgE antibodies attach to mast cells lining the airway or affected tissue.
- On subsequent exposure, the allergen binds IgE on mast cells, causing degranulation and histamine release.
Effects of Type 1 Hypersensitivity
- Histamine causes inflammation, vasodilation, increased vascular permeability (edema), itchiness, pain, and bronchoconstriction.
- Upper respiratory involvement: hay fever (sneezing, watery eyes, coughing).
- Gastrointestinal involvement: nausea, vomiting, diarrhea.
- Skin involvement: rashes and dermatitis.
- Lower airways: allergy-induced asthma.
- Systemic involvement can cause anaphylaxis.
Key Terms & Definitions
- Hypersensitive reaction — Excessive immune response to a harmless substance that causes inflammation and tissue damage.
- Type 1 hypersensitivity — Rapid allergic reaction involving IgE, mast cells, and histamine.
- Allergen — Substance that triggers an allergic reaction.
- IgE antibody — Immunoglobulin type E, binds allergens and activates mast cells.
- Mast cell — Immune cell that releases histamine during allergic reactions.
- Degranulation — Release of granules (e.g., histamine) from mast cells.
- Histamine — Chemical causing inflammation, vasodilation, and allergic symptoms.
Action Items / Next Steps
- Review the four types of hypersensitivity reactions.
- Read more about the roles of different immune cells in hypersensitivity.