Figure 1

The mammary gland epithelium is not a mucosal epithelium. A Mucosal surfaces colonized by bacterial communities deploy distinct protective mechanisms. Within the simple columnar epithelium, goblet cells secrete mucus that covers the mucosal surface. The mucus inner and outer layers retain high concentrations of secretory IgA and host antimicrobial peptides (amp) secreted by epithelial cells or specialized cells such as Paneth cells. M cells transport luminal antigens to the dendritic cells beneath. Plasma cells secrete sIgA. T and B cells are present in the lamina propria, some are associated in mucosa-associated lymphoid formations. B The mammary gland epithelium is devoid of mucus, and bovine milk contains low-concentration of IgG. A few macrophages and neutrophils can be found in the lumen, but neutrophils are recruited en masse by inflammation when bacteria proliferate in milk. Bacterial intruders are detected by the epithelium comprised of epithelial cells and intraepithelial dendritic cells. Macrophages and T cells are present in the lamina propria, but organised lymphoid formations are absent from healthy glands.