Which of the following substances serves as a physical and chemical barrier against pathogens, and why is it effective in preventing infections?

The skin serves as a physical and chemical barrier against pathogens. Its outer layer, the stratum corneum, is made of dead cells packed with keratin, making it difficult for microbes to penetrate. At the same time, the skin secretes sebum and sweat, which create an acidic environment that kills or inhibits bacteria and fungi. For example, a hand that is clean and intact will stop most germs from entering, reducing the chance of infection.