General Structure of FceRI and FceRII

Since an extensive review about FceRI has been recently published (9), we encourage its reading for detailed information and references about the molecular structure of the receptor and the mechanisms of FceRI-mediated signal transduction. 

Both FcRP and FcRy are involved in signal transduction. They both possess an ITAM motif, which allows them to bind SH2 domains, present in nonreceptor tyrosine kinase. Upon clustering of receptor-bound IgE by multivalent antigen, Lyn kinase, which is in part bound to FcRP independently of the ITAM motif, phosphorylates FcRP and FcRy ITAMS. This allows recruitment of additional Lyn by FcRp and of Syk kinase by FcRy. Then Lyn phosphorylates Syk, which in turn autophosphorylates. This leads to phosphorylation of downstream substrates by activated Syk. 

The receptor expressed by mast cells and basophils is essentially an aPy2 tetramer. In humans and in rat, but not in mouse, other cell types such as eosinophils, monocytes (3), dendritic cells (6), and Langerhans cells express a trimeric receptor lacking FcRp, although FcRp mRNA was detected in Langerhans cell from atopic patients (7) as well as in eosinophils (4). In the absence of FcRp, membrane-bound Lyn phosphorylates FcRy ITAM and Syk at a very low level. 

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Signal transduction through surface CD23 ligation is linked to cyclic nucleotides and nitric oxide (NO) pathways in various human cells and in rat macrophages (18). Some divergences are found between the signaUng pathways cou- 

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