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How does keratin in the outer layer of skin help block infection?

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Immune stimulation by activated helper T cells — immune system Stimulation of immune response by activated helper T cells. Activated by complex interaction with molecules on the surface of a macrophage or some other antigen-presenting cell, a helper T cell proliferates into two general subtypes, T_H1 and T_H2. These in turn stimulate the complex pathways of the cell-mediated immune response and the humoral immune response, respectively.

Keratin is a tough, insoluble, and fibrous structural protein that plays a critical role in the immune system by forming a primary external barrier against infection.12

Located in the outermost layer of the epidermis, known as the stratum corneum, keratin packs the cornified cells to provide structural integrity and a robust mechanical shield. This layer is highly stable due to disulfide bonds between its polypeptide chains, making it resistant to degradation by many proteolytic enzymes. By creating this dense, protective surface, keratin helps prevent pathogens and toxins from penetrating the body's underlying tissues. The effectiveness of this barrier is further supported by the constant renewal of skin cells from below, which ensures that the protective layer remains intact.3421