CD64, also known as FcγRI, is a high-affinity receptor for the Fc region of IgG antibodies and plays a crucial role in the immune response. It is primarily expressed on monocytes, macrophages, and dendritic cells, and can also be induced on neutrophils with IFN-γ and G-CSF. CD64 is involved in antibody-dependent cellular cytotoxicity (ADCC) and the clearance of immune complexes. The receptor's internalization process is a two-step mechanism: monomeric IgG binding leads to rapid internalization and recycling, while cross-linking causes retention and degradation of immune complexes within the cell. CD64's high affinity for IgG (in the nanomolar range) distinguishes it from other Fcγ receptors and is essential for initiating cellular effector responses even at low IgG concentrations. Structurally, CD64 consists of three extracellular Ig-like domains, a transmembrane region, and a cytoplasmic domain, with unique hinge angles that contribute to its high binding affinity. Recent studies have explored CD64 as a target for immunotherapeutic strategies in chronic inflammatory diseases, leveraging its role in macrophage activation and immune complex handling.
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