Functional properties of soluble CD21.

Soluble receptors may display immunoregulatory properties by blocking interactions between ligands and their membrane receptors or by triggering specific biologic responses through interaction with counter part membrane receptors. A natural soluble form of CD21 that is cleaved from lymphocyte membrane 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 fashion to IgE complexes, another ligand of CD23, the soluble CD21 was shown to efficiently trigger CD23-signalling pathways in human monocytes. By inducing release of proinflammatory cytokines and upregulating expression of molecules involved in antigen presentation, soluble CD21 modulates critical monocyte functions that may be relevant to allergic and inflammatory disorders.