Once infected, mammary gland innate immunity is initiated; bMECs act as the first line of defense against exogenous pathogens and secrete several cytokines (e.g., IL-1β, IL-8, and TNF-α) to recruit neutrophils into the mammary gland (8, 67, 68), causing a sharp rise in SCC intra-mammary infection in early- and late-lactation (69–71) and the corresponding mastitis phenotype. Here, TNF is linked to mastitis.