The initial breast milk, or colostrum, is much richer in antibodies than other secretions, because of its remarkably high concentration of SIgA (~12 g l-1) and SIgM (~0.6 g l-1). The individual variations are large, however, and the level decreases by a factor of approximately four after 2 weeks and then remains fairly stable throughout lactation (Goldman, 1993; Hanson et al., 1993). Antibodies of these two classes are produced locally by plasma cells as pIgA and pentameric IgM in the lactating breast (Brandtzaeg, 1983). Unoccupied pIgR molecules are also cleaved and released to the lumen as free SC (Fig. 14.2), which is present at a relatively high level in colostrum (~2 g l-1). Free SC exerts a stabilizing effect on SIgM (Brandtzaeg, 1975) and may by itself contribute to intestinal defence through inhibition of E. coli colonization and C. difficile toxin blocking, as mentioned above. When present in a bound form, SC may activate eosinophils (Lamkhioued et al., 1995), but it may counteract such pro-inflammatory stimulation in its free, soluble form (Motegi and Kita, 1998).
Because the lactating mammary glands are part of the integrated mucosal immune system (Fig.14.4), milk antibodies will reflect antigenic stimulation of MALT in the gut as well as in the airways, as mentioned earlier. The secretory antibodies are thus highly targetted against infectious agents in the mother's environment, which are those likely to be encountered by the baby during its first weeks of life. As mentioned previously, antibodies against various dietary antigens, such as gluten and cow's milk proteins, as well as against other potential allergens (Casas et al., 2000), are also often present in breast milk. The possible role of these IgA antibodies for the clinical presentation of immune-mediated adverse reactions to food in the infant will be discussed below.
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