New T-cell production by the thymus versus expansion of mature T cells. Each T cell recognizes a different target. In this figure, the diversity of targets is represented by different colors and letters of the alphabet. For instance, a T cell that recognizes target “A” may fight the flu virus, whereas a T cell that recognizes target “B” may fight a bacterial infection. T-cell diversity lost to HIV infection is not repaired by expansion of pre-existing mature T cells but can be improved by new synthesis of T cells in the thymus.