How we make immune cells that keep the body from attacking itself

A false-color scanning electron micrograph of a T cell.

NIH

One of the primary tasks of the immune system is to distinguish self from non-self. “Self” includes the normal proteins in the body, while "non-self" is a sign of potential harm, like a parasite, virus, or cancer cell. As the immune system’s T cells mature in the thymus (that's why they're called T cells), any cells unlucky enough to respond to "self" are killed off, lest they cause autoimmune diseases. This process is known as negative selection.

Like most biological processes, though, negative selection is not foolproof; not every autoreactive T cell ends up dying. To catch them, the body produces Treg (for T regulatory) cells, which provide another safeguard against autoimmunity. Their function is to keep the other T cells in check. However, the mechanism by which certain T cells mature into Treg cells is not yet well-defined. New research has identified how these specialized cells are produced, even though they respond to proteins normally found in the body.

All T cells express a receptor (creatively called the TCR, for T cell receptor) that helps them identify cells that are infected or defective. If this receptor ends up recognizing proteins that are found on healthy cells, it was thought that the T cell is induced to commit suicide before it can leave the thymus.