Abstract: A competitor for human Immunoglobulin E (IgE) comprises a polypeptide which has a core sequence of seventy-six amino acids which is shown, together with the corresponding DNA sequence coding therefor, in FIG. 2. This amino acid sequence, numbered 1 to 76, corresponds to amino acids 301 to 376 of the epsilon heavy cain of IgE. The 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 bind compete with native IgE for the high-affinity receptor sites on human cells. The polypeptide is indicated for the treatment of Type I hypersensitivity reactions such as hay fever. The polypeptide may be produced synthetically or by expression from Escherichia coli containing a plasmid having a DNA segment coding for the polypeptide.