Explanation: 👍🏻The correct answer is D. This is a phenomenon known as peripheral tolerance. It is an important factor because deletion of self-reactive T lymphocytes within the thymus ("central tolerance") is not completely efficient at removing all self-reactive T lymphocytes. Thus, one mechanism of peripheral tolerance is that of anergy: When a T lymphocyte receives the first signal (peptide-major histocompatibility complex) but no second signal (costimulation, such as CD28 -B7 ligation), that T lymphocyte undergoes a reprogramming known as anergy, wherein it is subsequently made refractory to any future stimulation. Note that an anergic T lymphocyte cannot be activated later even if costimulation is present.
👎🏻A is not correct. For a T lymphocyte to become activated and fully able to perform its effector functions, it must receive a second or costimulatory signal. Without a costimulatory signal, the T lymphocyte cannot be activated and instead becomes anergic.
👎🏻B is not correct. A T lymphocyte that becomes anergic does not undergo activation.
👎🏻C is not correct. Causing cells to undergo apoptosis is a function of an activated T-cytotoxic lympocyte.
👎🏻E is not correct. The T lymphocyte will not clonally expand. Clonal expansion is more typical of B lymphocytes and requires a costimulatory signal to first activate the lymphocyte.