PDCD1 and follicular lymphoma: In follicular lymphoma patients, PD-1+ T cells include both functionally “exhausted” (unable to produce cytokines) PD-1lo T cells and PD-1hi “non-exhausted” follicular helper T cells (CXCR5+BCL6+CD4+, supporting the growth and survival of B cells, and secreting IL-21 and IL-4) (30).