Integumentary System
The skin (epidermis over dermis over hypodermis), hair, nails, and glands — the body's first barrier, temperature regulator, vitamin D producer, and sensory surface.
As the first line of innate defense, intact skin physically blocks pathogens while its acidic secretions slow microbial growth. Melanocytes in the epidermis produce melanin, which shields deeper layers from ultraviolet damage.