Final answer:
IgE is the antibody class involved in allergic reactions by causing mast cells and basophils to release histamine upon secondary exposure to an allergen.
Step-by-step explanation:
The class of antibody that is involved in allergic reactions and causes mast cells and basophils to release histamine is IgE. This antibody binds to mast cells and upon secondary exposure to an allergen, the mast cells release histamines and other pro-inflammatory mediators that contribute to the symptoms of an allergy. Therefore, the matching of the antibody class to its description in the context of allergic reactions is B) IgE.