Which of the following statements about T lymphocytes is NOT true?

T lymphocytes, also known as T cells, play a crucial role in the adaptive immune response. They are primarily responsible for mediating cellular immunity, which involves the direct killing of infected cells by cytotoxic T cells and the coordination of immune responses by helper T cells.

The statement regarding their maturation site is significant. T lymphocytes do not mature in the bone marrow; instead, they originate from precursors in the bone marrow but migrate to the thymus gland to undergo maturation and differentiation into functional T cells. This critical step in their development is where they acquire the necessary receptors, such as CD4 for helper T cells and CD8 for cytotoxic T cells.

In contrast, the other statements accurately reflect the characteristics and functions of T lymphocytes. They indeed mediate cellular immune responses, cytotoxic T cells express CD8, and helper T cells express CD4 during their maturation process.