An extended family of Fc receptor relatives

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Abstract
A surprising number of Fc receptor (FcR) relatives have been recognized recently with the potential capacity to modulate innate and adaptive immune responses. The six human FcR homologs (FcRH1–6), which belong to a phylogenetically conserved gene family, have variable numbers of extracellular immunoglobulin domains of five different subtypes. FcRH immunoregulatory potential is implicated by the presence of consensus tyrosine‐based activation or inhibition motifs in their cytoplasmic tails. All but one of these new receptors, FcRH6, are expressed on B cells at different stages in differentiation. Their ligands, function, and prospective roles as diagnostic B cell markers and therapeutic targets are topics of intense interest.