Final answer:
IgE concentration in the blood is usually low but increases during allergic reactions. Mast cells bound with IgE degranulate upon exposure to allergens, leading to allergy symptoms.
Step-by-step explanation:
IgE is a type of antibody typically present in low concentration in the blood, with levels usually increasing during specific immune responses. IgE is most known for its role in allergic reactions, where its Fc region binds to an IgE-specific Fc receptor on mast cells' surfaces.
This binding makes mast cell degranulation highly specific and can lead to severe allergic reactions, including anaphylaxis, when exposed to specific allergens such as peanuts in a person with a peanut allergy. Therefore, the answer to the question 'IgE is in low concentration in the blood except during _______.' would be A) Allergic reactions. During an allergic response the IgE levels in the blood increase because the antibodies bind to allergens, causing mast cells to release histamines and other substances that lead to allergy symptoms.