Fate of antigen-activated small lymphocytes. A small resting (unprimed or naive) lymphocyte resides in the G_o phase of the cell cycle. At this stage, B and T lymphocytes cannot be distinguished morphologically. Following antigen activation, a B or T cell enters the cell cycle and enlarges into a lymphoblast, which undergoes several rounds of cell division and eventually generates effector cells and memory cells. Shown here are B-cell lineage cells.