The mechanism of the effect of omalizumab on hypereosinophilia is unclear; however, the defective expression of CD23 on B cells, as in our case, might be an explanation.1 The low-affinity receptor for IgE, FcɛRII, also known as CD23, is expressed on the B cells, monocytes, eosinophils, and Langerhans cells and plays an important role in the regulation of IgE production by B cells.2 This molecule is a transmembrane glycoprotein (type 2) of approximately 45 kDa and can be cleaved to yield a range of freely soluble CD23 (sCD23) fragments by the metalloprotease ADAM10. The best characterized ligands of CD23 are IgE and CD21, respectively. It was shown that antigen-IgE complexes are captured and internalized by CD23, which facilitate the antigen processing and presentation with major histocompatibility complex class 2 proteins.3 In addition, CD23 serves as a negative regulator of IgE production, which has been demonstrated through experimental animal studies in vivo. Increased production of IgE and defective antigen presentation were found in mice deficient in CD23.4 Accordingly, our case has defective expression of CD23 on B cells. Because of technical and financial reasons, we could analyze neither mutation of this molecule nor its soluble fragment in serum. We suggest that future studies investigating the role of CD23 (both its soluble fragments and expression on inflammatory cells, including B cells and eosinophils) and its mutation may provide some information about allergic and hypereosinophilic disorders and the mechanism of action of omalizumab on them.