The thymus is an important part of your child’s lymphatic system. It is an organ comprised of two lobes, which are encased in a capsule that sits behind the sternum and above the heart. A missing or malfunctioning thymus can make your child vulnerable to infections or developmental disorders.

Function

The thymus gland's main function is to bring a type of lymphocyte, or white blood cell called a T-lymphocyte, or T-cell, to maturity. The precursors of T-cells are produced in your child’s bone marrow. From the bone marrow, these cells migrate to the thymus, where they mature until ready to take up their role to fight infection. The thymus also manufactures thymosin, a hormone that stimulates lymphocytes in other organs to mature.

T-cell Role

The thymus releases adult T-cells into the bloodstream and they travel to other lymphatic organs to help fight disease and infection. T-cells work in tandem with B-lymphocytes, which serve as scouts that identify and attach themselves to foreign invaders, such as viruses, fungi or abnormal cells. T-cells travel to the targets the B-lymphocytes have identified and kill the invaders. B-lymphocytes do not have the capacity to kill other cells, and T-cells don't have the ability to identify targets for themselves. Your child needs both types of lymphocytes to fight infection.

Development

The thymus gland is large in infants and continues to grow through childhood. During this phase, the thymus matures more than enough T-cells to last throughout your child's life, so the gland begins to decrease in size at puberty. Fat slowly replaces the thymus until it becomes quite small in older adults. Because of this lifetime stockpile of T-cells, an adult can have his thymus removed with no ill effects, but a child who loses his thymus is vulnerable to infection.

DiGeorge Syndrome

DiGeorge syndrome, or congenital thymic hypoplasia, is a rare condition in which a missing portion of chromosome 22 causes a child to be born without a thymus or with one that is underdeveloped. If your child has this condition, she is vulnerable to infection. She also may have heart defects, a cleft palate, abnormal facial features and/or abnormally low calcium because of the missing portion of the chromosome. This condition has a high mortality rate, but in children who survive, the number of T-cells usually increases through childhood. Survivors, however, are likely to have learning disabilities and other physical developmental issues, according to the Encyclopedia of Children's Health 4.

Myasthenia Gravis

Under normal conditions, a person’s nerves transmit the command for a muscle to contract via the chemical acetylcholine. Myasthenia gravis is an autoimmune disease in which antibodies mistake acetylcholine receptors in the nerve endings for foreign invaders and destroy them. The condition is more common in adults, but it occurs in children as well. Researchers don’t fully understand the connection between the thymus and this condition, but they have discovered that almost all adults with myasthenia gravis have an abnormal thymus, according to The Myasthenia Gravis Foundation of California 5. Babies can be born with myasthenia gravis because of antibodies passed to them by their mothers, but it usually disappears within weeks or months of birth. In some rare cases, a defective gene may cause myasthenia gravis and in this case, it is present from birth. The primary symptom is intermittent muscle weakness, most commonly in the face, especially the eyes and mouth, but it also may affect the neck, legs, arms and hands.