Final answer:
Keratin plays a role as a protective barrier against mastitis by preventing the entry of pathogens into breast tissue.
Step-by-step explanation:
The role that keratin plays in regards to mastitis is that of a protective barrier. Keratin is a key component of skin and other epithelial tissues, providing a physical barrier against pathogens, including bacteria that can cause mastitis. When considering the options available, keratin does not serve as an inflammatory mediator, nutrient source, or antigen presentation, which are roles played by other components of the immune system. Particularly relevant in the context of mastitis, which is an infection of the breast tissue commonly associated with breastfeeding, the keratinized cells act as a frontline defense, preventing the entry and colonization of pathogens that could lead to infection.