A. Schematic diagram of the bovine mammary gland showing the most important anatomic factors that act as defense
barriers. The teat sphincter muscle represents the first line of defense, whereas the keratinized epithelium of the teat cistern is
considered the second line. B. Cellular and soluble factors that participate in the innate immune response of the mammary gland.
Macrophages located in the alveoli phagocytize bacteria that enter the mammary gland cistern. Activated macrophages release
cytokines such as TNF-a and IL-1b. C. Endothelial cells from blood vessels adjacent to alveoli express adhesion molecules in
response to pro-inflammatory cytokines; this, in turn, facilitates neutrophil recruitment from the bloodstream to the site of infection
in order to eliminate the invading bacteria.