T cells constitute an extremely heterogeneous population of lymphocytes that function to balance normal immune system homeostasis and adaptive responses to pathogens. T cells express unique heterodimeric T cell receptors (TCRs) that dictate antigen specificity and mediate appropriate responses to antigen stimulation.
Activated T cells express surface receptors such as CD25 (IL-2 receptor), CD71, and co-stimulatory molecules including CD26, CD27, CD28, CD30, CD154 (CD40L), and CD134 (OX40). In addition, activated T cells secrete a variety of cytokines that regulate and support the function of other immune cells. Transcriptional regulation of gene expression is critical throughout T cell development and transcription factors such as Foxp3, RORg, T-bet, and GATA-3 are required to both drive and modulate T cell differentiation and function. Flow cytometry is the most widely utilized method for analysis of T cell proliferation, as well as for phenotypic expression of surface markers, cytokines, and select transcription factors.