The Thymus 1961 – The Organ Responsible for T-Cell Maturation

Although the Greco-Roman physician Galen noted that the thymus gland, located just behind the breastbone at the front of the chest, changed size in adulthood, the function of this mysterious gland remained unknown until the 1960s. The mystery of the thymus never stopped people from enjoying cattle thymus (known as sweetbread) as food.

The Encyclopedia of Jewish Food  author Gil Marks writes: 

"Probably no community has embraced sweetbreads as food as much as the Ashkenazim  (German and Eastern European Jews), with perhaps only the liver being more popular among organ meats."

In 1961, French scientist Jacques Miller surgically removed the thymus glands of three-day-old mice and then observed a deficiency of certain lymphocytes (white blood cells) derived from this gland. Miller also observed that mice undergoing thymectomy (surgical removal of the thymus) became highly susceptible to infection and did not reject skin grafts taken from other mice – rejection being the expected response in mice with properly functioning immune systems.

Today, we know that the thymus has two regions: an outer cortex and an inner medulla. Pre-T cells  are produced in the bone marrow and travel through the bloodstream to the thymus. These cells enter the cortex, where a sequence of molecular events enables them to recognise particular antigens – substances that trigger the production of antibodies that allow foreign invaders to be destroyed. During this maturation process, cells that respond inappropriately to normal tissue components are eliminated. The surviving cells enter the medulla and eventually pass into the bloodstream, from where they can protect the body against potentially harmful agents. The maturation of T cells is controlled by hormones produced by the thymus.

The two-lobed thymus reaches its maximum size during puberty and then begins to shrink dramatically. By the age of 75, the thymus becomes difficult to distinguish from the surrounding fatty tissue. Loss of the thymus early in life can lead to T-cell deficiency, severe immune deficiency and high rates of infection.

Description of the article image: An artistic representation of a T cell attacking a cancer cell. A T cell (an infection-fighting white blood cell) can kill certain cancer cells and some virus-infected cells. T cells take their name from the thymus, the main organ responsible for T-cell maturation.

Historical figures mentioned in the article: Galen of Pergamon (129 – 199), Jacques Francis Albert Pierre Miller (b. 1931)

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