Absolute neutrophil count (ANC) >= 1,500/mcL within 14 days prior to step 2 randomization Absolute neutrophil count (ANC) >= 1,500/mcL within 28 days prior to registration Within 14 days prior to registration: Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL within 28 days prior to registration Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcl documented within 28 days prior to step 1 registration Absolute neutrophil count (ANC) >= 750/mcl within 28 days prior to registration absolute neutrophil count (ANC) ?1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL SAFETY RUN-IN: Within 10 days of registration: Absolute neutrophil count (ANC) >= 1,500 /mcL RANDOMIZED PHASE II CLINICAL TRIAL: Within 10 days of registration: Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) < 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL. Within 14 days of treatment registration: Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) > 1000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL, performed within 28 days of study registration Absolute neutrophil count (ANC): >= 1500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 750/mcL; a subject will not be excluded because of pancytopenia related to disease Absolute neutrophil count (ANC) >= 1,500/mcL Within 28 days of cycle 1 day 1: Absolute neutrophil count (ANC)\t>= 1,000/mcL Within 5 days prior to the start of study treatment: Absolute neutrophil count (ANC) >= 1,500 /mcL. Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL. Absolute neutrophil count (ANC) >= 1.5 k/mcl, obtained within 7 days prior to first study treatment Absolute neutrophil count (ANC) >= 1,500 /mcL. Within 7 days of registration: Absolute neutrophil count (ANC) >= 1,500/mcL Within 10 days prior to on-study date: Absolute neutrophil count (ANC) ? 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 750 /mcL Absolute neutrophil count (ANC) ? 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/ mcL Absolute neutrophil count (ANC) >= 1,000 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Obtained within 14 days prior to enrollment: absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1500/mcL, performed within 14 days of protocol registration Absolute neutrophil count (ANC) > 1000/mcL Absolute neutrophil count (ANC) >= 500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL, performed within 14 days of registration Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL. Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL, during screening and on cycle 1, day 1 Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC): >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL within 14 days prior to registration Absolute neutrophil count (ANC) > 500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Performed 28 days prior to study registration up to the first dose of study drug: absolute neutrophil count (ANC) >= 1,000 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) < 1000/mcl Absolute neutrophil count (ANC) >= 1,500/mcL PHASE I: Absolute neutrophil count (ANC) > 1,500/mcl PHASE II: Absolute neutrophil count (ANC) > 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL except in cases of marrow infiltration by lymphoma Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Performed within 28 days prior to study registration up to the first dose of study drug: Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,250/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Newly diagnosed MCL: Absolute neutrophil count (ANC) > 750/mm^3; patients who have bone marrow infiltration by MCL are eligible if their ANC is equal to or > than 500 Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) < 1000/mcL Patients must have absolute neutrophil count (ANC) > 1000/mcl Absolute neutrophil count (ANC) ? 1,000/mcL OR ? 750/mcL if neutropenia due to iBCL Absolute neutrophil count (ANC) >= 1,500 /mcL Within 14 days prior to registration: Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500 /mcL REGISTRATION TO TREATMENT (STEP 1): Absolute neutrophil count (ANC) >= 1,500 /mcL, within 14 days prior to first dose of pembrolizumab REGISTRATION TO TREATMENT (STEP 2): Absolute neutrophil count (ANC) >= 1,500 /mcL, within 14 days prior to first dose of pembrolizumab Absolute neutrophil count (ANC) >= 1500 /mcL Absolute neutrophil count (ANC) ? 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >=1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Within 14 days of registration: Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000 cells/mcL within 42 days prior to registration Absolute neutrophil count (ANC) greater than or equal to 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) < 1.0 K/mcL at time of enrollment Absolute neutrophil count (ANC) greater than or equal to 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) greater than or equal to 1,500/mcL (ANC >= 1.5 x 10^9/liter [L]) Within 14 days prior to registration: Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,000/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,000/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) greater than or equal to 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL, performed within 14 days prior to registration Performed within 28 days of radiation therapy: Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1000/mcL Absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) > 1000/mcL Absolute neutrophil count (ANC) >= 1.5 K/mcL Absolute neutrophil count (ANC) > 1500/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1500/mcL Absolute Neutrophil Count (ANC) ? 1,500/mcL Absolute neutrophil count (ANC) of at least 2000 per mcL Absolute neutrophil count (ANC) count greater than or equal to 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) < 1000/mcl An absolute neutrophil count (ANC) >= 1,500/mcl Absolute neutrophil count (ANC) > 1500/mcl Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) > 750 cells/mcl Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) greater than or equal to 1,500/mcl Absolute neutrophil count (ANC) < 1.5 K/mcL Absolute neutrophil count (ANC) >= 1,000/mcL Absolute neutrophil count (ANC) >= 1,500/mcL within 28 days prior to registration Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) < 1.0 K/mcL At the discretion of the operating surgeon, absolute neutrophil count (ANC) > 1000/mcl Absolute neutrophil count (ANC) < 1.0 K/mcL At the discretion of the physician, absolute neutrophil count (ANC) > 1000/mcl Absolute neutrophil count (ANC) < 1.0 K/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500 /mcL Absolute neutrophil count (ANC) >= 1,500/mcL Absolute neutrophil count (ANC) >= 1,500 /mcL, performed within 10 days of protocol enrollment Absolute neutrophil count (ANC) >= 1,500 /mcL