Disclosed is an alpha chain of the human high-affinity IgE receptor (FceRIa), wherein the amino acid lysine at position 43 (K43) is exchanged with an amino acid selected from the group consisting of alanine, serine, tyrosine, isoleucine, leucine, asparagine, aspartic acid, methionine, phenylalanine, glutamic acid, threonine, glutamine, tryptophan, glycine, and valine, preferably alanine, glycine, serine or tyrosine, especially alanine.
