What is one of the main functions of the skin in immune defense?

The skin serves as a crucial barrier to pathogens, playing a vital role in the immune defense system. It acts as the first line of defense against external threats such as bacteria, viruses, and other harmful microorganisms. The outermost layer of skin, known as the epidermis, consists of tightly packed epithelial cells that form a protective physical barrier, preventing pathogens from entering the body. Additionally, the skin produces antimicrobial substances such as defensins and lysozymes that further enhance its protective function.

Through this barrier function, the skin helps to maintain homeostasis and protects internal tissues and organs from infection and disease. This property of the skin is critical for the overall immune response, as breaches in the skin can lead to an increased risk of infections. While the other options describe various functions of the body's systems—such as nutrient absorption, energy storage, and blood transport—none of these directly relate to the skin's primary role in immune defense.