The half life of IgG in serum is typically longer (1-3 weeks) than that for IgA or IgM (1-2 days), and the half-life of IgG2 is slightly longer than for IgG1 [122] . Evidence suggests that IgG2 has a higher affinity for FcRn than IgG1 [122] . In bovine colostrum, IgG1 is many fold greater in concentration than IgG2 [82] , although they are of approximately equal concentrations in serum. It may be that the majority of the IgG2 taken up by the mammary epithelial cell during colostrum formation is not passed on to the alveolar lumen, but rather is recycled back to the extracellular fluid. The FcRn is thought to have a major role in the recycling of IgG in various tissues in the body [130] [131] [132] . That is, IgG that potentially may be lost through various tissues is recycled by the respective cells by binding to FcRn and recycled back to the blood or lymph. This is supported by studies of overexpression of FcRn in transgenic mice where there is an extension of the half-life of serum IgG [133, 134] , as well as a boosting of the overall humoral immune response of the mice [135] .