then proceeds to a common lymphoid progenitor (CLP), which can only differentiate into T, B, or NK cells. These CLP cells then migrate via the blood to the thymus, where they engraft:. Henceforth they are known as thymocytes, the immature stage of a T cell.

The earliest cells which arrived in the thymus are commonly termed double-negative, as they express neither the CD4 nor CD8 co-receptor. The newly