allergic reaction

Meaning: An allergic reaction is a hypersensitivity, an immune response to a harmless antigen such as pollen or a food protein. In the immediate (type I) form, the first exposure is called sensitization. Helper T cells and B cells respond, and plasma cells secrete allergen-specific IgE. This IgE binds receptors on mast cells and basophils, and no symptoms appear yet. On re-exposure, the allergen cross-links the bound IgE, and the cells degranulate and release histamine. Histamine causes vasodilation, higher capillary permeability and smooth muscle contraction. The result is redness, swelling, itching, mucus and airway narrowing. Widespread release can cause anaphylaxis, with a sharp fall in blood pressure.

On the MCAT: A passage may describe a person with no symptoms on first contact but a strong reaction on the second, and ask you to explain it. The answer is IgE-primed mast cells, which reflects immune memory. You may be asked which cell type releases histamine, or why antihistamines relieve symptoms (they block histamine receptors). A contrast case is delayed-type (type IV) hypersensitivity, such as poison ivy, which appears a day or more later and is mediated by T cells rather than antibodies.

Common confusion: Students assume no symptoms on first exposure means no immune response. Sensitization happens silently, and it sets up the reaction on the next exposure. Also, the antibody here is IgE, not IgG, and the harm comes from the host's own response, not from a pathogen.