Macrophage receptors for IgE: binding of IgE to specific IgE Fc receptors on a human macrophage cell line, U937.

Human IgE was found to bind to the human macrophage line U937. Binding was specific for IgE in that it was inhibited by 3 different human IgE myeloma proteins and by Fc fragments prepared from IgE, but not by IgE Fab fragments or by myeloma proteins representative of the 4 IgG subclasses or by a rat IgE myeloma protein. Dissociation and association rates were rapid, with estimated t1/2 of less than 1 min. The Ka was 4.15 X 10(7) liter/mol. Between 53,000 and 93,000 sites per cell were calculated. The U937 receptor for IgE was trypsin sensitive, whereas the receptor for IgG on the same cell was relatively trypsin resistant.