Which structure serves as the body's first, continuous, and impassable barrier against most pathogens, killing or inactivating them through drying and acidity?

1. Before any immune factors are triggered, the skin functions as a continuous, impassable barrier to pathogens. 2. Pathogens are killed or inactivated on the skin by desiccation, or drying out, and by the skin's acidity. 3. Beneficial microorganisms on the skin also compete with invading pathogens to prevent infection. 4. The spleen, thymus, and bone marrow are all involved in immune cell development or filtering, not in this first physical barrier role. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 42 "The Immune System", section Physical and Chemical Barriers_