Flow cytometry provides the ability to type immune cells based on their phenotype. The presence of specific cell surface markers, cytokine expression, or phosphorylation of key proteins may be used to immunophenotype specific sub-populations from a heterogeneous starting population. Thus, the unique signature carried by different immune cells can be utilized to isolate cell populations in a similar way to how the immune system targets foreign pathogens: antibody binding.
T lymphocytes may be immunophenotyped on the basis of CD3+ expression and subsequently further sub-divided (e.g. CD8+ Killer T cells and CD4+ Helper T cells). Moreover, T cell phenotypes are flexible and can adapt to different microenvironments; phenotypes may also overlap among multiple T cell populations. Further T cell subset classification may be based on specific cytokine secretion in response to certain stimuli or the phosphorylation of immune signaling proteins, such as STAT proteins. Therefore the ability of the T cell to respond to the environment, such as in a disease state, can be monitored using flow cytometry.