Ligands and biological activities of soluble Fc gamma receptors.

Soluble forms of low-affinity Fc gamma receptors (sFc gamma R) circulate in biologic fluids. Their plasmatic levels vary in immunological disorders or related diseases. They are produced by enzymatic cleavage of the membrane receptors or by alternative splicing. They bind IgG with the same isotype specificity as their cell surface counterparts and thus modulate Fc-dependent immune functions. Recent data suggest that they also bind non-Ig ligands present on leukocytes. Functional implications of these findings are discussed.