J

Figure 1 Fc receptor structures. Each of the Fc-y receptors, the FcaR and the « chain of FceRI are members of the immunoglobulin gene superfamily. FceRI I is a member of the C-type animal lectin family.

and eosinophils. On large granular lymphocytes (LGL)/natural killer (NK) cells, these receptors mediate efficient non-major histocompatibility complex(MHC)-restricted cytotoxicity of virus infected and tumor cells. Platelets respond to FcyR cross-linking by mediator release and expression of adhesion-related activation antigens, resulting in platelet aggregation. B cell FcyRs are involved in the regulation of cell activation and immunoglobulin production. In addition to these beneficial effects, the FcyRs are also involved in autoimmune diseases. IgG autoantibody-coated red cells, platelets, etc., are removed through interactions with FcyRs on tissue macrophages.

Distribution and properties of FcyR (Table 1)

FcyRIa, the only isoform of CD64 that has been characterized at the protein level, is a 72 kDa glycoprotein which is recognized by monoclonal antibodies (tnAbs) 32, 22, 197 and 10.1. Although Fc7RIb2 mRNA is coexpressed with the FcyRIa message, corresponding proteins have not been identified for this or the FcyRIbl and FcyRlc transcripts. FcyRI is the receptor with the highest affinity for monomeric IgG, having an equilibrium dissociation constant (Kd) of 10 8 to 10"9 m for human IgGl and IgG3, and mouse IgG2a and IgG3. FcyRI is therefore occupied by IgG under physiologic conditions. It is expressed constitutively by only monocytes, macrophages and dendritic cells, but cytokines can induce its expression on granulocytes. FcyRIa requires associated y chains for stable expression and activation of phagocytic signaling, but can appar

Table 1 Properties of the Fc receptors

Receptor

Mol. wt (kDa)

Affinity

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