A new concept of triggering mechanisms of IgE-mediated histamine release

It is generally accepted that an initial step of reaginic hypersensitivity reactions is a bridging of mast cell-bound IgE antibody molecules by antigen. Since IgE molecules are firmly bound to receptors on mast cells, bridging of cell-bound IgE molecules probably brings receptor molecules into close proximity. A hypothesis was therefore presented that such a local change in membrane structure and/or possible interaction between adjacent receptor molecules may be triggering mechanisms of IgE-mediated histamine release. The hypothesis was tested by use of antibodies against "exposed portion" of receptor molecules on rat basophilic leukemia cells. It was found that antireceptor antibodies and its F (ab′)₂ fragments induced noncytotoxic histamine release from normal rat mast cells without participation of IgE, while the monovalent Fab′ fragments of the antibody failed to do so. However, sensitization of normal rat skin with the Fab' fragments followed by an intravenous injection of antirabbit IgG induced skin reactions. These findings support the concept that bridging of receptors rather than polymerization of IgE molecules is responsible for the activation of membrane-associated enzymes which in turn leads to histamine release.