Immunological events at the surface of basophil granulocytes and mast cells which induce degranulation.

IgE molecules combine with basophil granulocytes and mast cells through the Fc portion and sensitize these cells for reaginic hypersensitivity reactions. The number of receptors for IgE per human basophil granulocyte is estimated to be 40,000 to 100,000 and those on rat mast cells is about 300,000. The binding of IgE with the receptor is reversible and does not involve covalent bonding. The affinity of IgE molecules for the receptors is high; the equilibrium constant of the reaction is in the order of 10(9)/mole for both human basophils and rat mast cell systems. Such a high affinity will explain why a minute amount of IgE antibody can sensitize the target cells and why sensitization with the antibody is so persistent. The initial step of the reaginic hypersensitivity reactions is probably bridging of cell-bound IgE molecules by antigen, which activates enzymatic sequences. The release of chemical mediators, however, is controlled by pharmacological factors which have most important rôles in transducing immunological signals into intracellular signals and subsequent biochemical processes.