In cHL, however, most tumor cells do not express HLA class I due to loss of beta2‐microglobulin.20 As revealed by the CheckMate 205 trial (ClinicalTrials.gov identifier: NCT02181738),7 HRS cell expression of β2‐microglobulin and HLA class I molecules was not predictive of the response to nivolumab.21 Intriguingly, HRS cell expression of HLA class II molecules was instead predictive of complete remission in the same study.21 This finding suggests that CD4 + T cells play a role in mediating the response to PD‐1 blockade via an alternative, HLA class II‐dependent mechanism. Here, PDCD1 is linked to classic Hodgkin lymphoma.