All of the following are true of IgE except that it

• A. fails to fix complement.
• B. is heat stable.
• C. attaches to tissue mast cells.
• D. is found in the serum of allergic persons.

Answer: (B)

IgE mediates immediate allergic reactions by binding to high-affinity Fc receptors on tissue mast cells and basophils, so that when an allergen cross-links the bound IgE, granules are released and mediators like histamine are released. It does not fix complement because its Fc region does not efficiently activate the classical complement pathway. It is found in the serum of allergic persons, though usually at low levels unless there is strong atopy or allergic activity. Importantly, IgE is heat-labile; heating can denature it and abolish its ability to bind antigen and trigger degranulation. Therefore, stating that IgE is heat stable is not correct.