They always say that a strong defense is a good offense. The teat end is the first line of defense in fending off mastitis infections in a dairy herd. The teat end provides a block or barrier against entry of mastitis-causing bacteria into the teat cistern, I have referred to it as an offensive rather than a defensive line.
Somatic cells are the second line of defense.
The teat end's sphincter muscle, a ring of smooth muscle surrounding the teat canal, functions to keep the teat canal closed, prevent milk from escaping, and prevents bacteria from entering the teat. This keratin forms a barrier against bacteria. The ball is snapped as the first teatcup is applied. During milking, bacteria may be present near the opening of the teat canal. Bacteria are propelled into the teat canal and teat cistern when there is admission of undesired air into the milking unit.