It should be remembered that colostrum and milk not only contain immunoglobulins, but also contain a range of antimicrobial factors and factors that may impact the immune system [10, 154, 160, [162] [163] [164] [165] [166] [167] [168] . These include the iron-binding antimicrobial protein lactoferrin, antibacterial enzyme lactoperoxidase, antibacterial and lytic enzyme lysozyme, oligosaccharides that function as analogues of microbial ligands on mucosal surfaces, antimicrobial heat stable peptides (defensins), and soluble CD14. In addition, colostrum and milk contain leukocytes, including activated neutrophils, macrophages and lymphocytes. Colostrum also contains cytokines and growth factors that may affect neonatal intestinal development, as well as intestinal immune responses to disease in adults [166, 169] . The relative concentrations of these factors vary considerably among species. Furthermore, colostrum provides a source of energy which may impact IgG absorption in the neonate [170] , and provide additional energy for an effective immune response.