Intravenous immunoglobulins (IVIg) have immunomodulatory effects in vivo and are widely used in the treatment of autoimmune diseases, such as idiopathic thrombocytopenic purpura (ITP). The mechanisms by which IVIg can prevent platelet clearance in ITP patients are not fully understood but are known to require the participation of low affinity Fcγ receptors (FcγRs), which interact poorly with monomeric immunoglobulin G (IgG). Given the importance of low affinity FcγRs in the treatment of ITP, we hypothesized that immune complexes (IC) produced in vitro could reproduce the effects of IVIg. Small‐size tetramolecular IC were prepared using mouse monoclonal anti‐human IgG and human Fc fragments. The effects of tetramolecular IC and IVIg on the in vitro and in vivo inhibition of phagocytosis of opsonized blood cells were compared. The results obtained showed that tetramolecular IC were at least six times more efficient than IVIg to prevent phagocytosis of opsonized red blood cells in vitro, and clearance of platelets in the thrombocytopenic mouse model.