The Neonatal IgG receptor FcRn
The FcRn molecule (neonatal Fc receptor) is responsible for the transport of immunoglobulin from mother to neonate. In the case of some animals (eg. rats, mice and ruminants) the receptor is expressed in the gut of the newborn and transports IgG from the colostrum to the blood during the first 24 hours of life. In humans the receptor transports IgG across the placenta. It also seems from recent studies that this same receptor is responsible for maintaining the long plasma half-life of IgG. The FcRn molecule consists of an alpha chain (shown in red) and which is homologous to MHC class-I molecules, associated with beta-2-microglobulin (shown in yellow). Data suggests that two FcRn molecules transport one IgG molecule as shown above. The receptor binds to the interface between the CH2 and CH3 domains of the IgG heavy chains in the Fc of the IgG molecule. The conserved carbohydrate which is shown in green does not appear to be essential for the FcRn binding function, experiments showed that aglycosylated IgG seems to be transported normally and also still retains a long half-life.
You might also like to compare this complex of FcRn and IgG with that of MHC class-I and the immunoglobulin like T-cell receptor . You will see that a different interface between the two molecules is used in each complex.
For more information see the following review
Mike Clark.