Fc receptors play a major role in immune defenses against pathogens and in inflammatory processes.
Because of their ability to bind the Fc portion of antibodies, Fc receptors provide a link between humoral and cellular immune responses. There are Fc receptors for each Ig isotype.
FcRs fall into two general classes: those involved in effector functions and those that transport immunoglobulins across epithelial barriers.
Except for FcΕRII (CD23), which is C-type lectin-like, all other known Fc receptors are members of the immunoglobulin superfamily .
FcγRI and FcΕRI are high-affinity receptors with dissociation constants ranging from 10-8 to 10-10 M, whereas the other receptors, such as FcγRII and FcγRIII, are low-affinity receptors with dissociation constants ranging from 10-5 to 10-7.
FcγRIIB (CD32), the low-affinity receptor for the Fc fragment of IgG, is expressed on almost all hematopoietic cells, including B lymphocytes, NK KIR*cells,etc.
The recent crystal structures of a human FcΕRIa , FcγRIIa , FcγRIIb  and FcγRIII  revealed a conserved immunoglobulin-like (Ig-like) structure, particularly the small hinge angle between the two Ig-like domains, that is unique to the Fc receptors.
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