Thymus

You have a special part in your chest.

It sits behind your breast bone. It is in front of your heart. This part helps your body stay well.

It teaches tiny cells how to fight. These cells learn to find bad things. They also learn not to fight your own body.

This part is very busy when you are young. It grows as you grow. It gets smaller as you get older.

By the time you are a teen, it changes. It starts to turn into fat. It still works to keep you safe.

The thymus is a special organ in your chest.

It sits behind your breast bone. It is also in front of your heart. This organ helps your immune system work. The immune system is the way your body fights germs.

The thymus has a very important job. It helps T cells grow and learn. T cells start as young cells called thymocytes. These cells come from your bone marrow. Inside the thymus, they go through two tests.

First, they go to the outer part called the cortex. Here, they undergo positive selection. This means they learn to find the right targets. Next, they go to the inner part called the medulla. Here, they undergo negative selection. This test makes sure they do not attack your own body.

The thymus is largest when you are a child. It can weigh up to 50 grams by puberty. After that, it begins to shrink. This shrinking is called involution. As it shrinks, it is replaced by fatty tissue. Even as an adult, your thymus still helps make some T cells.

The thymus is a vital organ in your immune system. It helps your body adapt to specific foreign invaders. This organ is located in the upper front part of your chest. It sits behind the sternum, which is your breast bone. It also sits in front of your heart. The thymus is made of two lobes that meet in the middle.

Inside the thymus, young cells called thymocytes go through a training process. These cells come from your bone marrow. They must pass two important tests to become mature T cells. First, they go to the outer cortex for positive selection. In this step, they learn to react with MHC immune receptors. This ensures they can recognize the targets they need to fight.

The next step happens in the inner part called the medulla. Here, the cells undergo negative selection. This test makes sure the T cells do not attack your own body proteins. If a cell reacts too strongly to your own tissue, it is eliminated. This way, the mature T cells are safe to enter your blood. These cells then go out to provide vital immune functions.

People have known about the thymus since the time of the Ancient Greeks. However, scientists only learned its true function in the 1960s. The thymus is largest and most active during childhood. At birth, it is about 4 to 6 cm long. By the time you reach puberty, it may weigh 40 to 50 grams. This is when the organ reaches its maximum size.

After puberty, the thymus begins to shrink in a process called involution. The organ gets smaller and less active as you get older. Fatty tissue gradually replaces the original thymus tissue. By old age, the thymus may only weigh 5 to 15 grams. Even as an adult, some T cell development still happens. This shrinking process is seen in most animals with a thymus.

The thymus is a specialized primary lymphoid organ within the immune system. Its main purpose is to facilitate the maturation of T cells. These cells are vital to the adaptive immune system. This system allows the body to adapt to specific foreign invaders. The thymus is located in the anterior superior mediastinum. This is the upper front part of the chest. It sits behind the sternum and in front of the heart.

The organ is composed of two lobes that meet at the upper midline. Each lobe is covered by a protective capsule. Inside, the thymus is divided into two distinct regions. The outer region is called the cortex. The inner region is known as the medulla. These regions contain a network of epithelial cells. These cells support immature T cells, which are called thymocytes. Other cells like macrophages and dendritic cells are also present.

Thymocyte maturation is a precise, step-by-step process. It begins when hematopoietic precursors migrate from the bone marrow into the thymus. Once inside, these cells undergo V(D)J recombination. This genetic process creates unique T cell receptors. The cells then move into the cortex for positive selection. In this stage, thymocytes must interact with MHC immune receptors on epithelial cells. If a cell cannot react to the MHC, it undergoes apoptosis, or programmed cell death.

After passing positive selection, cells move to the medulla for negative selection. This step is crucial for preventing autoimmune diseases. In the medulla, epithelial cells and dendritic cells express proteins from throughout the body. A gene called AIRE helps stimulate this expression. Thymocytes that react too strongly to these "self" antigens are eliminated through apoptosis. This ensures that mature T cells do not attack the body's own tissues. Some cells that survive this stage become T regulatory cells.

As T cells mature, they express either CD4 or CD8 proteins, but not both. A cell that binds mostly to MHC class I becomes a cytotoxic CD8 positive T cell. A cell that binds mostly to MHC class II becomes a CD4 positive T cell. Once fully mature, these cells emigrate from the thymus. They enter the peripheral circulation to provide cell-mediated immunity. This process is aided by hormones and cytokines like thymulin and thymosins.

The thymus changes significantly throughout a person's life. At birth, it is about 4 to 6 cm long and 1 cm thick. It grows until puberty, reaching a mass of approximately 40 to 50 grams. After puberty, the organ undergoes thymic involution. This is a process where the thymus decreases in size and activity. Fatty and connective tissues gradually replace the functional thymic tissue. By old age, the thymus may weigh only 5 to 15 grams.

This process of involution is linked to rising levels of sex hormones. It is an evolutionary process seen in most vertebrates with a thymus. While the thymus shrinks, some T cell development continues into adulthood. However, abnormalities can cause serious health issues. Problems with thymus development can lead to severe immunodeficiency. For example, DiGeorge syndrome involves a failure of the third and fourth pharyngeal pouches to develop. This can result in a lack of a functional thymus.

Historically, the thymus has been known since the time of the Ancient Greeks. However, its specific role in the immune system was not clear until the 1960s. Medical issues can also arise from growths in the tissue. A thymoma is a type of cancer of the thymus tissue. Lymphoma is a cancer arising from immature lymphocytes. Doctors may sometimes perform a thymectomy, which is the surgical removal of the organ. Understanding the thymus helps us understand how our bodies defend themselves.