Mechanisms for sensitization of human basophil granulocytes

It was found that the number of cell bound IgE molecules per basophil was 10,000-40,000. The number of receptors for IgE was 30,000-90,000. The binding of IgE molecules with the receptor is reversible and no covalent bond is involved in the binding. When basophils were exposed to pH 4.0 or lower, cell bound IgE completely dissociated from the receptor. At physiological condition, IgE molecules avidly bound with the receptor. The association constant of the binding reaction was estimated to be 10⁸ to 10⁹M^-1. The high association constant is probably responsible for a long persistence of passive sensitization with IgE antibody. The number of IgE molecules on basophil surface has biological significance. An inverse relationship was observed between the number of cell bound IgE molecules and the optimal concentration of anti IgE for maximal histamine release. The results suggested that the number and affinity of cell bound IgE antibody determines the sensitivity of the cells to antigen. Basophil membrane appears to be in a liquid state and surface immunoglobulin is movable at 37°C. When the cell bound IgE on basophils reacted with divalent anti IgE, IgE molecules formed patches and eventually migrated into one pole of the cells. The monovalent Fab fragments of the antibody combined with the cell bound IgE but failed to induce redistribution of the cell bound IgE. It was found that cap formation was not involved in the process of histamine release. However, both histamine release and cap formation required divalent anti IgE, indicating that bridging of cell bound IgE molecules initiates both processes. Upon cap formation by anti IgE, receptor molecules and IgE migrated as a complex. Evidence was obtained that not only IgE bound receptors, but also free receptors, migrated to the same pole upon capping. The results strongly suggested that a group of receptor sites is associated with each other at the cell membrane, or that receptor molecules are multivalent with respect to the combining sites for IgE.