Structure and Function of Fc ε Receptor II (Fc ε RII/CD23): A Point of Contact Between the Effector Phase of Allergy and B Cell Differentiation

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Abstract
The Fcε receptor II (FcεRII/CD23) has been proposed to have multiple functions as a membrane‐bound or soluble molecule: a function in B cell growth and differentiation and a role in the effector phase of IgE‐mediated immunity. We recently demonstrated the presence of two forms of FcεRII (FcεRIIa and FcεRIIb) whose structures differ only at their N‐terminal cytoplasmic regions. The regulatory mechanisms of their expression strongly suggest that FcεRIIa and FcεRIIb function in B cells and in the effector cells of IgE‐mediated immunity, respectively. To elucidate the function of soluble FcεRII/CD23 (sFcεRII) the recombinant soluble molecule was produced. This recombinant receptor could competitively block the IgE binding of eosinophils, monocytes and even basophils and could inhibit the IgE‐mediated function of effector cells such as monocytes. These findings suggested that sFcεRII could competitively regulate the function of effector cells in IgE‐mediated immunity and that the recombinant sFcεRII could be applied clinically for the control of allergic reactions. The expression of FcεRII on FcεRII‐negative B and T cell lines by cDNA transfection resulted in homocytic aggregation. The function of FcεRII on B cells as an adhesion molecule was also demonstrated.