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

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.

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.