Question

which part of the immune system is a physical barrier?
skin
tears
mucus
stomach acid

Explanation:

皮肤是免疫系统的物理屏障，它覆盖身体表面，阻挡病原体入侵。眼泪、黏液是黏膜分泌物，胃酸是化学物质，它们属于其他免疫防御机制。

Answer:

skin