Final answer:
IgE is the immunoglobulin responsible for mediating the sensitization of mast cells. It plays a pivotal role in allergic responses by inducing mast cell degranulation upon allergen exposure.
Step-by-step explanation:
The immunoglobulin whose main function is to mediate sensitization of mast cells is IgE. IgE antibodies are bound to mast cells via their Fc portion, and when an allergen binds to these antibodies, the crosslinking of IgE induces degranulation. This degranulation causes the mast cells to release histamines and other modulators that are responsible for the symptoms of allergic reactions. IgE's role as an allergy mediator is best exemplified when, on first exposure to an allergen, an IgE antibody is synthesized by plasma cells. Upon secondary exposure, the bound IgE triggers mast cell degranulation to release inflammatory mediators.