A research laboratory is investigating the molecular requirements for mast cell activation in Type I hypersensitivity. Laboratory-grown human mast cells are sensitized with IgE antibodies specific for a synthetic hapten. The cells are then exposed to a monovalent form of the hapten containing only a single epitope. Despite confirmed binding of the hapten to IgE molecules, no degranulation occurs as measured by absence of histamine and tryptase release. The researchers repeat the experiment using a bivalent hapten with two identical epitopes, which results in immediate degranulation with release of histamine and tryptase.
Which of the following best explains why degranulation did not occur with the monovalent hapten?