The B-cell binding site on human immunoglobulin E

IMMUNOGLOBULIN E comprises the main immunoglobulin class associated with allergy. Its multifarious activities are mediated by two types of Fc receptors found on different cell populations, FcεR1 on mast cells and basophils, and FcεR2 on inflammatory cells (monocytes, eosinophils and platelets) and B lymphocytes. Recombinant ε-chain fragments synthesized in Escherichia coli have provided the means of mapping the receptor-binding sites on human IgE, and blocking IgE-receptor interactions. We have previously shown that the FcεR1 binding site is contained within a sequence (Gln 301-Arg 376) spanning the Cε2 and Cε3 domains. Here we show that FcεR2 can recognize a motif in the Cε3 domain that is formed on dimerization of one or both of the flanking (Cε2 and Cε4) domains. Glycosylation of IgE is not required for the activity of either receptor.