Final answer:
Lymph nodes are not a surface barrier and instead are part of the body's second line of defense, unlike the epidermis, mucous membranes, and cilia which serve as mechanical barriers in the first line of defense.
Step-by-step explanation:
The component among the options given that is not a surface barrier serving as the first line of defense is lymph nodes. Surface barriers include the epidermis, mucous membranes, and cilia. These act as mechanical barriers and are part of the immune system's first line of defense against pathogens. The epidermis, which is the outermost layer of skin, is tough and contains keratin that makes it difficult for pathogens to penetrate. Mucous membranes produce mucus which is a sticky substance that can trap pathogens, and the cilia, which are small hair-like structures, help move the mucus along with any trapped particles out of the body. In contrast, lymph nodes are part of the immune system's second line of defense and play a role in filtering harmful substances and containing immune cells that can respond to pathogens.