Table 1.

Theoretical reasons why type 3 immunity should contribute to MG defense against pathogens.

Features of type 3 immunity	Features of mammary gland defenses	Ref
Immunity to extracellular bacteria and fungi	Infection by extracellular bacteria	[61]
Defense of epithelial barriers	Mainly epithelial infection (“duct disease”)	[62]
Amplifies neutrophilic inflammation	Neutrophils main cell type recruited during mastitis	[63]
Neutrophils important effector arm of type 3 immunity	Neutrophils main immune defense of the mammary gland	[63]
Induces epithelial self-defense by antimicrobial peptides	Mammary epithelial cells produce AMPs in response to bacteria or cytokines	[62]
Targets epithelial cells to trigger inflammation (chemokines)	Mammary epithelial cells respond to IL-17A by secreting chemokines	[17, 24]
Signature cytokines: IL-17A, IL-17F, IL-22	IL-17A, IL-17F, IL-22 in mastitic milk	[19, 23]
Targets epithelial cells through receptors to IL-17 and IL-22	Mammary epithelial cells express IL-17R and respond to IL-17A & IL-17F	[24]
Immunization elicits CD4 + cells producing IL-17 (Th17 lymphocytes)	CD4 + IL-17A + cells correlate with vaccination or antigen-specific sensitization of the mammary gland	[33]
The IL-23/IL-17 axis drives granulopoiesis	Mastitis drains neutrophil reserves	[64]