This image illustrates one way to distinguish different cells with identical morphologies, by labeling them with antibodies directed against cell-specific markers.
Shown here is a suspension of thymic lymphocytes stained with red fluorescent antibodies against the CD4 antigen, a marker of helper-inducer type T-cells, and green fluorescing antibodies against CD8, present on cytotoxic-suppressor type T-cells. The majority of T-cells in the thymus are immature cells which express both molecules ("double positives") and therefore appear yellow. T-cells in peripheral lymphoid tissue (lymph node, spleen, etc.) are mostly mature cells which would express only one or the other of these markers.