. "The change in morphology and behavior of a mature or immature T cell resulting from exposure to a mitogen, cytokine, chemokine, cellular ligand, or an antigen for which it is specific, leading to the initiation or perpetuation of an immune response." . . . "T cell activation involved in immune response" . "T cell activation during immune response" . "T lymphocyte activation during immune response" . "T-cell activation during immune response" . "T-lymphocyte activation during immune response" . . . . . . . . .