Question

establish a relation between allergy and histamine

Answer 1

Allergy is an exaggerated response to innocuous substances. It is caused due to hypersensitivity of a person to these specific substances called allergens. In majority of the instances when foreign proteins enter the body, the T-cells recognise them to be harmless and prevent the B-cells from responding. However, in allergic persons, the B-cells start the production of huge quantities of IgE antibodies to counter the allergen. The IgE attaches itself to the receptors on the mast cells. Inside the mast cells are granules containing histamine. This is how the body becomes sensitive to the allergen.
When the body encounters the allergen the next time, the allergen binds itself with IgE antibody bound mast cells. This causes the mast cells to rupture releasing histamine.
Typical symptoms of allergy are caused due to the release of histamine. It increases the permeability of the capillaries, contraction of smooth muscles, secretion by mucous glands, and dilates small arteries but not veins resulting in increased blood flow (seen as redness of the affected area) cause fluid accumulation and swelling.