CD23, also known as Fc epsilon RII (FceRII), is a low affinity receptor for IgE and belongs to the C-type lectin superfamily. It is expressed on mature B cells, activated macrophages, eosinophils, follicular dendritic cells, and platelets. Soluble CD23 is generated through cysteine-protease or metalloprotease activity. A long (a) form is expressed consitutively on B cells, while a short (b) form is upregulated by IL-4 during allergic inflammation. Fragments of different sizes are generated and function may be related to fragment size, as well as form. CD23 plays a role IgE regulation, isotype switching, B cell homeostasis and stimulating monocytes to release pro-inflammatory cytokines.