Hypersensitivity Reactions Quiz

See Type I Hypersensitivity. Option A is correct. Type I hypersensitivity is the immediate, allergic type of reaction. On the first exposure to an allergen (a harmless substance such as pollen or peanut that provokes allergy), the immune system makes allergen-specific IgE antibodies, which attach to mast cells and basophils (cells packed with chemical mediators). This is the sensitisation step, and nothing is felt. On a later exposure, the allergen cross-links the IgE on the cell surface, which makes the cell release its contents (degranulation). The mediators include histamine, which widens blood vessels and makes them leaky, and leukotrienes, which tighten the airways. The result appears within minutes, usually within about 30 minutes, and can be local, such as hay fever or urticaria, or body-wide, as in anaphylaxis. A helpful reminder is the ABCD method, in which type I is the Allergic type. Whenever you see IgE, mast cells and an immediate onset, think type I.

• Type I is Allergic (immediate, IgE and mast cells)
• Type II is antiBody-mediated (antibodies against cell-bound antigens)
• Type III is immune Complex-mediated (antibodies with soluble antigens)
• Type IV is Delayed (T-cell mediated)

• Type: type I is the immediate, allergic type of hypersensitivity.
• Mechanism: on re-exposure the allergen cross-links IgE on mast cells, which degranulate.
• Mediators: histamine widens and leaks blood vessels and leukotrienes tighten the airways, within minutes.