The other major classes of immunoglobulins transported into colostrum and milk are IgA and IgM. Immunoglobulin A is the major immunoglobulin in human colostrum and milk (Figure 1 ), however it is also present in milk of most other species. Colostrum and milk IgA and IgM are found in the form of secretory IgA, or sIgA, and sIgM. Much of these are produced by plasma cells in the mammary tissue. The plasma cells are part of the gut-associated lymphoid tissue (GALT), the largest immune organ of an organism, which includes the Peyer's patches, lymphoid and myeloid cells in the lamina propria and intraepithelial lymphocytes [146, 147] . Lymphocytes from the GALT system migrate to the mammary gland and provide a direct link between the antigen exposure response in the mother's mucosal immune system, especially via the enteric mucosal immune system, and the secretory immunoglobulin repertoire of the mammary gland [18] . This means that maternal colostrum and milk will contain antibodies specific for pathogens that may be encountered by the neonate's intestine and other mucosal tissues [10, 18, 148] , providing a rationale for the observations summarized above that bovine colostrum from nonimmunized cows also may afford passive immune protection against human pathogens [54, 55] .