Abstract: A polypeptide competitor or analogue for human Immunoglobulin E (IgE) low affinity sites comprises a polypeptide which has a sequence of amino acid which has a sequence of amino acids which is shown in Table I. This amino acid sequence corresponds to amino acids 340 to 439 of the epsilon heavy chain of IgE. A particularly preferred polypeptide competitor has a sequence of amino acids corresponding to amino acids 340 to 547 of the epsilon heavy chain of IgE as set out in Table V herein, which also shows the corresponding DNA sequence coding therefor. Such a polypeptide may also include additional short sequences at the beginning and/or end of the core sequence which are physiologically harmless and do not contribute to the ability of the core sequence to compete with native IgE for the low affinity receptor sites on human cells. The polypeptide may be produced synthetically or by expression from Escherichia coli containing a plasmid having a DNA segment coding for the polypeptide.