A protein that transports antibodies across the placenta, and that saves them from degradation in adults. It is related to the MHC class 1 protein family (these proteins have a crucial function in the immune systems defence against virusses and tumor cells).
FcRn is important in two ways. First it makes sure that the antibodies your body needs to defend itself against a whole range of infections don't get digested too quickly. Secondly because unborn fetuses don't make antibodies yet, so right after birth they are very vunerable to infections. In order to give them some protection, the mothers body actively transports some of her own antibodies across the placenta into the fetus's bloodstream, where they protect the baby in its first months.
This is the way it works: First picture a cell that is in the side of a vein. The surface is never quite still, its always taking up little bits of its surroundings, in this case the blood that flows past it. Now in these little sips it takes, there are all kinds of proteins. Most of these will be taken to special compartments to be degraded, but some are just too valuable to waste that way. One of these highly valuable proteins is IgG (immuneglobuline G), the most abundant type of antibody we have. Now once this sip is taken inside the cell, the pH drops from 7.5 to about 6, which means that the interior of the sip will become a little bit acidic. As this happens, FcRn, (which sits in the side wall of the sip) changes a little bit (Histidine residues are protonated) so that it suddenly really likes IgG. All the IgG in the sip will attach to the FcRn in the wall, and as the unbound content of the sip is taken to so-called lysosomes to be digested, the FcRn with its attached IgG will return to the cellsurface. There the pH will rise again from 6 to 7.5 which will change FcRn again a little bit (deprotonate the histidine residues) so that the IgG lets go and returns to the bloodstream.
The second role, the transport of IgG from mother to baby across the placenta, is done in almost the same way. The only difference is that instead of the FcRn with its bound IgG going back to the surface it came from, it now is directed to the other side of the cell, where it ends up releasing the IgG into the fetal bloodstream instead.
Some (but very, very few) people lack FcRn. These people are usually very vunerable to infections, and need to be watched very carefully during pregnancy.