Functional properties of soluble CD21

Soluble receptors may display immunoregulatory properties by blocking inter actions between ligands and their membrane receptors or by triggering speci fic biologic responses through interaction with counter part membrane recep tors. A natural soluble form of CD21 that is cleaved from lymphocyte membra ne CD21 circulates in normal human serum. Soluble CD21 retains the capacity to bind iC3b and CD23, the known ligands of membrane CD21. In a similar fa shion to IgE complexes, another ligand of CD23, the soluble CD21 was shown to efficiently trigger CD23-signalling pathways in human monocytes. By indu cing release of proinflammatory cytokines and upregulating expression of mo lecules involved in antigen presentation, soluble CD21 modulates critical m onocyte functions that may be relevant to allergic and inflammatory disorde rs. (C) 1999 Elsevier Science B.V. All rights reserved.