T cells play an integral role in adaptive immunity and are broadly defined based on expression of the T cell receptor (TCR), CD4, CD8, and NK markers. TCR is a dimer consisting of alpha and beta (α/ß) chains or gamma and delta (γ/δ) chains. Over 90% of T cells in the periphery express TCRα/ß, which are further divided by the presence of absence of costimulatory receptors CD4 and CD8.

Autoimmune lymphoproliferative syndrome (ALPS) is a rare disorder of impaired lymphocyte homeostasis that leads to autoimmunity and increased risk of lymphoma. Patients with ALPS typically present in the early years of life with nonmalignant lymphadenopathy, hepatosplenomegaly, and autoimmune cytopenias. One of the hallmarks of ALPS is expansion of CD4/CD8 double negative (DN) T cells that express TCR α/ß. TCRα/ß+ DN T cells normally constitute less than 1% of peripheral blood and lymph node T cells but are increased in patients with ALPS.