The human low affinity receptor for IgE (Fc e RII), also known as the leukocyte differentiation antigen CD23 [1], is a type II integral membrane glycoprotein of 45 kDa which is constitutively expressed on resting, β + /δ + B lymphocytes and is lost after isotype switch. Fc e RII/CD23 is also found on follicular dendritic cells, eosinophils, platelets, alveolar macrophages and can be induced on monocytes and Langerhans cells by IL-4. Two isoforms (CD23a and CD23b) differing in the N-terminal cytoplasmic part have been identified by molecular cloning. Soluble IgE binding fragments (sCD23) are released from CD23+ cells by proteolytic cleavage. Pleiotropic activities have been proposed for the Fc e RII/CD23 and its soluble derivatives. Although specific functions in IgE regulation have been described, the biological role of the CD23 molecule remains elusive. Two recently recognized characteristics of the Fc e RII/CD23 will be presented: 1) specific induction of CD23 on T-cells by allergens and 2) focusing of antigen via CD23.