The programmed death-1 (PD1) protein is expressed by follicular helper T-cells and is useful in the diagnosis of T-cell lymphomas and in highlighting L&H cells in Hodgkin lymphoma. Anti-PD-1 antibody has been used as anti-tumor immunotherapy agents. This effect is possibly due to the targeting of PD1 expressing tumor-infiltrating lymphocytes which can be enumerated by this antibody.