Location and Structure

• The thymus lies directly beneath the sternum in the middle of the chest at the same level as the heart. It has a pyramidal shape and is divided into two lobes that separate at the mid-line of the body. The thymus primarily consists of two types of cells, lymphocytes and reticulin fibers, similar to other organs of the lymphatic system.
  
  

T-lymphocyte Production

• White blood cells protect the body from infection and sickness and consist of many specialized cells, including lymphocytes. Some lymphocytes produced in the bone marrow during pregnancy migrate to the thymus where they proliferate, differentiate and mature into T-lymphocytes. During this process, they learn how to discern self-cells from foreign cells, so they do not attack the body when released into the blood. They are referred to as T-lymphocytes because they develop in the thymus, as opposed to B-lymphocytes, which mature in the bone marrow.
  
  

Function in Immunity

• T-lymphocytes do not react to viruses or other foreign invaders until the thymus releases them into the bloodstream. Once mature, some T-lymphocytes travel and settle in other lymphatic organs, such as the lymph nodes, spleen and appendix, while others circulate through the blood. The presence of foreign cells activates the T-lymphocytes into action. Depending on the type of T-lymphocyte (cytotoxic, helper or suppressor), they assist in killing foreign cells, activating other immunity cells or ending the immune response after the invader has been sequestered or eliminated.
  
  

Development, Growth and Regression

• The thymus develops early during pregnancy and grows rapidly until birth. It reaches its greatest size during infancy and continues to grow until puberty, though at a much slower rate. Beginning at puberty, the thymus begins a process of regression. It shrinks in size as fibrous tissue and fat begin to replace the lymphoid tissue. As the size of the thymus decreases, so does its function in producing T-lymphocytes.
  
  

Associated Diseases

• Athymia, the absence of the thymus, results in a lack of T-lymphocytes, which in turn leads to immune deficiency. T-lymphocytes that do not properly learn how to distinguish self-cells from foreign cells attack the body if released into the bloodstream, resulting in an autoimmune reaction or disease, such as myasthenia gravis. Another associated disease includes thymus cancer but it is rare. Individuals with lupus, rheumatoid arthritis or myasthenia gravis are more likely to develop thymus cancer, according to the U.S. National Library of Medicine.