The immune system ( 免疫系统 ), which protects the body from infections, diseases, and other injuries, is composed of the lymphatic system ( 淋巴系统 ) and the skin. Lymph nodes ( 淋巴结 ), which measure about 1 to 25 centimeters across, and small vessels called lymphatics ( 淋巴细管 ) compose the lymphatic system. The nodes are located in the groin ( 腹股沟 ), armpits ( 腋窝 ) , throat, and trunk ( 躯干 ), and are connected by the lymphatics. The nodes work with the body’s immune system to fight off infectious agents like bacteria and fungus ( 真菌 ). When infected, the lymph nodes are often swollen and sensitive. The skin, the largest organ of the human body, is also considered part of the immune system. Hundreds of small nerves in the skin send messages to the brain to communicate pressure, pain, and other sensations. The skin encloses the organs to prevent injuries and forms a protective barrier that repels ( 抵制 ) dirt and water and stops the entry of most harmful chemicals. Sweat glands ( 汗腺 ) in the skin help regulate ( 调节 ) the body’s temperature, and other glands release oils that can kill or impede ( 阻止 ) the growth of certain bacteria. Hair follicles ( 毛囊 ) in the skin also provide protection, especially of the skull ( 颅骨 ) and groin. (189 words)