Figure 1.

(A) The multiple functions of FcRn are dependent on its ability to sort monomeric IgG away from lysosomal degradation within cells and release bound cargo during exocytic events at plasma membrane. (B) The fact that FcRn salvage pathway is saturable is a well-known phenomenon referred to as fractional catabolic rate and caused by the fact that the pool of FcRn available in cells to recycle or transport its ligand can be limited. Thus, when FcRn is fully saturated, the unbound ligand is cleared, primarily through lysosomal degradation. (C) The prolonged IgG half-life results of the transgenic mice that overexpress FcRn clearly suggest a correlation between the levels of expression of FcRn and the protection of IgG.