T cells and their subsets can be defined by differential expression of cell surface markers including CD3, CD4, CD8, and CD25. Using panels of directly conjugated fluorescent antibodies to these specific markers, multicolor flow cytometric analysis allows researchers to interrogate the levels of multiple markers simultaneously on individual cells. This can provide information about the cell lineage and state of differentiation of cell subsets in a particular sample. Adding markers such as CCR7, CD62L, or CD69 to an analysis provides important information about the potential for cells to home and localize within the body, as well as the activation status of the T-cell subset of interest.
By analyzing the expressed patterns of various markers—including not only cell surface receptors, but also cytokine secretion profiles and intracellular signaling molecules—researchers have defined phenotypes that represent functionally distinct T-cell subsets (eg, Th1, Th2, Th17, Treg, Th9).