Lymphoid organs mainly include the thymus, spleen, tonsils and lymph nodes, etc. Lymphoid organs can be divided into two categories, central lymphoid organs and peripheral lymphoid organs, depending on their occurrence and function. Lymphoid organs are lymphoid tissue-dominated organs that realize immune functions in the body, so they are also called immune organs. The thymus gland is located in the upper part of the chest cavity, above the heart. The spleen is located in the upper left part of the abdominal cavity and is the largest lymphatic organ in the body, and the tonsils are located at the base of the tongue and around the pharynx. All of these lymphoid organs are made up of lymphoid tissue and function similarly to the lymph nodes in that they all produce lymphocytes.