Which organs are involved in your immune system and how they fight off infections.

The immune system is made up of a network of cells, tissues, and organs that work together to protect the body. White blood cells, also called leukocytes (say: LOO-kuh-sytes), are part of this defense system.

When the body is fighting infection, lymph nodes can become enlarged and feel sore. Spleen: The largest lymphatic organ in the body, which is on your left side, under your ribs and above your stomach, contains white blood cells that fight infection or disease.