Recognizing the “Life Clock” in Your Body

  Many people think that the thymus gland, like the appendix and wisdom teeth, is a dispensable degenerative organ. Actually that’s not the case. Where is the thymus? The thymus is shaped like a butterfly, and this “butterfly” lies quietly in the upper part of the anterior mediastinum behind the sternum. Let’s get to know it together.
The thymus is the “production floor” of T cells

  The thymus is an important immune organ in the human body, and it is the place where T cells differentiate, develop and mature. The main role of the thymus is to “educate” T cells, teaching them to recognize “which markers or antigens in the body are native and which are foreign.” In addition, the thymus can also secrete thymic hormones and hormonal substances.
  The thymus first appears in the 9th embryonic week and matures in the 20th embryonic week. The thymus weighs 15 to 20 grams in the neonatal period, increases rapidly after infancy, and reaches a peak (30 to 40 grams) in puberty. After puberty, the thymus gradually shrinks and degenerates with age. In old age, the thymus shrinks significantly and is replaced by adipose tissue, and its function decline leads to a decline in the body’s immune function. Some people liken the thymus to the “life clock”, which is the earliest organ in the human body to begin to age.
  The main function of the thymus is to induce T cell differentiation and development and to develop immune tolerance to self-antigens. It begins with the entry of bone marrow lymphoid stem cells into the thymus with the blood to become progenitor T cells. Under the action of thymic hormones and various cytokines in the thymic microenvironment, the progenitor T cells become CD4+CD8+ double-positive pre-T cells with low expression of CD3. Some of these cells undergo “positive selection” in the thymic cortex to become CD4+ or CD8+ single-positive T cells. After “negative selection” at the junction of thymic cortex and medulla, some are involved in the formation of central immune tolerance against self-antigens, and some develop into CD4+ or CD8+ single-positive mature T cells. Such cells that have not yet received antigen stimulation are also referred to as naive T cells.

What are the diseases of the thymus

  The thymus can also get sick. The diseases related to the thymus are mainly divided into the following categories:
  1. Thymic tumors: Thymic tumors include thymoma and thymic carcinoma. Thymic carcinoma is the most common malignant tumor of the anterior mediastinum, most of which are located in the anterior superior mediastinum. When the diaphragm descends during the embryonic stage, part of the thymus tissue is brought to the inferior mediastinum, so some tumors can be located in the anterior inferior mediastinum. 20 to 50 years old, rare before the age of 20.
  Thymoma is mainly composed of thymic epithelial cells and lymphocytes. Patients may be asymptomatic and are mostly found during X-ray examination. Dull chest pain, shortness of breath and cough are the most common symptoms. Severe pain, superior vena cava occlusion syndrome, diaphragmatic paralysis, and hoarseness suggest extensive tumor invasion. About 20% to 30% of patients with thymoma have thymus-related immune diseases, the most common of which is myasthenia gravis. X-ray films show a round or oval mass shadow with clear, sharp or lobulated margins in the anterior superior mediastinum. On the lateral radiograph, the density is light and the outline is not very clear. This disease should be considered. CT or MRI examinations can help to understand the size and extent of the tumor. Relying on a large number of surgical and clinical data, the thoracic surgery team of our hospital was the first in the world to propose that thymoma is different from thymic carcinoma and rarely has lymph node metastasis. Myasthenia gravis is rarely associated with thymic carcinoma.
  2. Benign thymic diseases: benign thymic diseases include thymic cysts, thymic hyperplasia, etc. Most patients with thymic cysts have no obvious clinical symptoms, more than those found during physical examination. For small and asymptomatic thymic cysts, surgery is not required. Surgical excision is recommended for patients with large thymic cysts that compress peripheral nerves, organs, and tissues and cause related clinical symptoms, or patients with high risk of cyst rupture and intracystic hemorrhage. For patients with pure thymic hyperplasia, no surgical treatment is required. For patients with thymic hyperplasia combined with thymus-related immune diseases, further evaluation is required.
  3. Thymus-related immune diseases: The thymus is an important immune organ in the human body and is closely related to various autoimmune diseases such as myasthenia gravis, systemic lupus erythematosus, Sjögren’s syndrome, Good’s syndrome, dermatomyositis, and autoimmune hepatitis. . Among them, myasthenia gravis is the most common thymus-related autoimmune disease. Myasthenia gravis is mainly divided into five types: ocular muscle type, systemic type, acute severe type, delayed severe type, delayed severe type, and muscular atrophy type. Most of the patients initially had symptoms such as easy fatigue, ptosis, and blurred vision, and showed the characteristics of “heaviness in the morning and evening”. 10% to 15% of patients with myasthenia gravis have thymoma, and about 80% have thymic hyperplasia. For patients with thymoma, surgery should be performed as soon as possible to prevent tumor progression. Patients with thymic hyperplasia should seek medical attention as soon as possible, and ask neurologists and thoracic surgeons to comprehensively evaluate whether surgery is needed. Some patients’ symptoms can be significantly improved or even disappeared after surgery.
How to prevent and treat thymus disease early

  Thymus-related diseases are preventable. Many of the preventive measures are the same as the key points for preventing other diseases.
  1. Stay away from high-risk factors such as alcohol, tobacco, and ionizing radiation.
  2. Reasonable diet, regular three meals, control total calorie intake, strengthen exercise, enhance physical fitness, and improve self-immunity.
  3. Regular physical examination, check chest CT.
  4. If you have symptoms such as cough, chest pain, stridor, recurrent respiratory tract infection, dyspnea, hoarseness, etc., you should go to the thoracic surgery department in time.
  5. If there is ptosis, blurred vision, and general weakness, after excluding ophthalmic diseases, the hospital should be considered for relevant examinations for myasthenia gravis, and chest CT should be improved.
  6. In the event of myasthenia gravis, red blood cell dysplasia, hypogammaglobulinemia, polydermatomyositis, systemic lupus erythematosus, rheumatoid arthritis, thyroiditis or Good’s syndrome, be sure to go to the thoracic surgery department to complete the chest. CT, excluding thymoma.