# An Insight into Probiotics Bio-Route: Translocation from the Mother's Gut to the Mammary Gland
## 4 Microbial Transmission: Solving the Labyrinth Path
### Cross-Contamination from Infants' Oral Cavity
#### 4.3.2 Mucosal Sampling and Migration of Dendritic Cells  
The pathway of transmitting maternal microbiota from the gut to breast milk (entero-mammary pathway) can be linked to the improvement of the immunity in newborns [45; 87]. The mother passes down some of her immunoglobulin (Ig) to the developing fetus to acquire passive immunity during the pregnancy period. IgG is the only antibody that can cross the placental barrier in humans through endosomes within the syncyotrophoblasts of the placenta [88; 89]. After giving birth, the maternal immunity is still being successfully transferred and passive immunity is achieved as the lactating mammary glands takes over the functionality of the placenta [89]. Breast-feeding maintains a strong interaction between the mother and her infant by continuing the transfer of maternal immunoglobulin molecules into the newborn. SlgA is the predominant class of Ig found in HBM and is released by the antibody-secreting cells found in the mammary glands. However, the mammary gland does not seem to stand alone because it is highly influenced by the antibodies produced by the mother in her own gut [90]. During the late pregnancy and lactation stage, the maternal IgA antibody-secreting cells, found in the gut and in the respiratory system, are translocated into the mammary glands [91; 92]. The gut mucosal epithelial chemokine CCL28 is the key regulator of the build-up of IgA antibody-secreting cells. The build-up of these cells occurs in the lactating mammary gland. A study using BALB/c mice as the animal model revealed that blocking of CCL28 can lead to significant accumulation of IgA plasma cells in lactating mammary glands [93]. Therefore, the data suggest an internal pathway to transport these antibody-secreting plasma cells from the maternal gastrointestinal tract to her mammary glands, without involving blood circulatory vessels. Furthermore, the translocation is regulated by specific carrier cells [51]. It has been recognized for many years that human lymphatic vessels exhibit a high capacity for accumulating particulate material. The human lymphatic system not only drains the lymph, but is also responsible for moving the immune cells of lymphocytes and antibodies throughout the body to help initiate and participate in an immune response. Several blind ended vessels and fine capillaries of the lymphatic system are found throughout the human body [94; 95]. Gut associated lymphoid tissue (GALT) is the main lymphatic present around the gastrointestinal tract (Figure 3) comprising of Peyer's patches, isolated follicles, and mesenteric lymph nodes [94; 96]. The GALT serves as antigen sampling and inductive sites of the mucosal immune system. SlgA cells from the gut can adhere selectively to specific M cells found in the intestinal Peyer's patches. This will mediate the translocation of the SIgA cells from the intestinal lumen to GALT, and finally to mammary glands via the entero-mammary link [97]. The migration of SIgA cells from the gastrointestinal tract to mammary glands is believed to be under the regulation of hormones such as prolactin, estrogen, and progesterone [98]. Migration of antibody-secreting cells such as SIgA to HBM, originating from the maternal gut, could clearly explain the existence of lymphatic systems and the entero-mammary pathway [91]. However, the sampling and translocation of maternal gut microorganisms into breast milk is best explained by the dendritic cells (DCs). These cells are found throughout many sites including intestinal tissues. DCs actively migrate between the intestinal lamina propria, GALT, Peyer's patches, isolated lymphoid follicles, and in the mesenteric lymph nodes [97; 99]. The main function of DCs is to sample antigens found in the intestinal lumen, then migrate into lymphatic nodes and present antigens into T cells to induce their proliferation and differentiation [99].