Phagocytic Cell Fc Receptors

Following interactions with the endothelium, the neutrophils express a number of specific receptors including Fc receptors for immunoglobulins (Table 1). Three classes of receptors are expressed by phagocytic cells: FcyRI (CD64), FcyRI (CD32), and FcyRIII (CD16). Most receptors are hetero-oligomeric complexes consisting of nine membrane-associated FcRs and three soluble FcryRs coded by eight genes. The receptors are transmembrane proteins with similarities to the BCR and TCR recognition proteins. Some receptors have "immu-noglobulin-like" domains. High-affinity FcyRI (CD64) contains three immu-

Tablet Cellular Receptors on the Surface ofPhagocytic Cells

Receptor

Location

Ligand

CR1 (CD35)

Activated PMNs

C3b, iC3b, and C4

Follicular dendritic cells

CR2 (CD21)

Follicular dendritic cells

iC3b and C3dg

CR3, Mac-1, CDllb/CD18

Monocytes

iC3b

Activated PMNs

Dendritic cells

FC7RI (CD64)

Monocytes/macrophages

Human IgGl and IgG3

Activated PMNs

Mouse IgG2a

Oxidative burst

Fc^RII (CD32)

Monocytes/macrophages

Polymeric IgG

Activated PMNs

Respiratory burst

FC7RIII (CD16)

Macrophages

IgG2a and IgG3

noglobulin-like domains in the extracellular region. FcyRII and FcyRIII have only two immunoglobulin domains and a lower binding affinity for IgG (Deo et al., 1997).

Genetic polymorphism among the Fc receptors is common. Alternate forms ofFcyRII (CD32) differby a single amino acid (arginine or histidine) at position 131 in the second Ig domain creating FcyRIIa-R131 or FcyRIa-H131(Fig. 1). The low-affinity receptor FcyRIIIalso has two alternate forms: FcryRIIIa and FcyRIIIb. Subtypes of both the FcyRIIIa and b receptors have been identified. The two allotypes of FcyRIIIa contain either valine or phenylalanine at position 158. Allotypes of FcyRIIIb-NAl or NA2 differ in five nucleotides that result in different glycosylation patterns (Deo et al., 1997)

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