Inclusion Criteria:

          -  Have pathologically confirmed iNHL, MBC or HNSCC

          -  Lymphoma subtypes that may be enrolled include small lymphocytic lymphoma

        Exclusion Criteria:

          -  Is currently participating and receiving an investigational therapy (not standard
             therapies) or has participated in a study of an investigational agent and received
             study therapy or used an investigational device within 28 days of the first dose of
             treatment.

          -  Any patients that require immediate treatment or cytoreduction are excluded. Note:
             This is applicable for iNHL, MBC, and HNSCC populations.

          -  Any patient with transformed lymphoma, or patients with grade 3A follicular lymphoma
             are excluded.

          -  MZL patients with gastric MALT lymphomas with disease localized to the stomach are
             excluded, and any patient with disease in a site where injection is determined to be
             high risk.

          -  Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any
             other form of immunosuppressive therapy within 7 days prior to the first dose of trial
             treatment. Patients on chronic steroids (more than 4 weeks at stable dose) equivalent
             to ≤ 10mg prednisone will not be excluded.

          -  Hypersensitivity to pembrolizumab, poly-ICLC, Flt3L or any of their excipients.

          -  Has had a prior anti-cancer monoclonal antibody (mAb) within 28 days prior to study
             D22 (First dose of pembrolizumab) or who has not recovered (i.e., ≤ Grade 1 or at
             baseline) from AEs due to agents administered more than 28 days before initiation of
             in situ vaccine protocol.

          -  Has had prior chemotherapy, targeted small molecule therapy, or RT therapy within 14
             days prior to study D0 or who has not recovered (i.e., ≤ Grade 1 or at baseline) from
             AEs due to a previously administered agent. Note: Patients with chronic ≤ Grade 2 AEs
             such as neuropathy are an exception to this criterion and may qualify for the study.

          -  Has received prior radiotherapy within 2 weeks of start of study treatment.
             Participants must have recovered from all radiation-related toxicities, not require
             corticosteroids, and not have had radiation pneumonitis. A 1-week washout is permitted
             for palliative radiation (≤2 weeks of radiotherapy) to non-CNS disease.

          -  If patient received major surgery, they must have recovered adequately from the
             toxicity and/or complications from the intervention prior to starting therapy.

          -  Has known active central nervous system metastases, leptomeningeal disease and/or
             lymphomatous meningitis. Patients with previously treated brain metastases may
             participate provided they are stable (without evidence of progression by imaging for
             at least 28 days prior to the first dose of trial treatment and any neurologic
             symptoms have returned to baseline), have no evidence of new or enlarging brain
             metastases, and are not using steroids for at least 7 days prior to trial treatment.
             This exception does not include leptomeningeal disease or lymphomatous meningitis,
             which are excluded regardless of clinical stability.

          -  Has active autoimmune disease that has required systemic treatment in the past 1 year
             (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs). Replacement therapy (e.g. thyroxine, insulin, or physiologic corticosteroid
             replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a
             systemic treatment. Patients on chronic steroids (more than 4 weeks at stable dose)
             equivalent to ≤ 10mg prednisone will not be excluded.

          -  Has a history of (non-infectious) pneumonitis that required steroids or has current
             pneumonitis.

          -  Has an active infection requiring systemic therapy at time of enrollment in trial.

          -  Has a known additional malignancy that is progressing or requires active treatment.
             Exceptions include basal cell carcinoma of the skin or squamous cell carcinoma of the
             skin that has undergone potentially curative therapy or in situ cervical or anal
             cancer, prostate cancer on stable dose of hormonal therapy without rising PSA.

          -  Has a history or current evidence of any condition, therapy, or laboratory abnormality
             that might confound the results of the trial, interfere with the patient's
             participation for the full duration of the trial, or is not in the best interest of
             the patient to participate, in the opinion of the treating Investigator.

          -  Has known psychiatric or substance abuse disorders that investigator believes would
             interfere with cooperation with the requirements of the trial.

          -  Is pregnant or breastfeeding, or expecting to conceive or father children within the
             projected duration of the trial, starting with the pre-screening or screening visit
             through 120 days after the last dose of trial treatment (roughly two and a half years
             after enrollment).

          -  HIV positive with detectable viral load, or anyone not on stable anti-viral (HAART)
             regimen.

          -  Has known active Hepatitis B (e.g., HBV detected by PCR) or active Hepatitis C (e.g.,
             HCV RNA [qualitative] is detected).

          -  History of allogeneic hematopoietic cell transplantation or solid organ
             transplantation.

          -  Has received a live vaccine within 30 days prior to the first dose of study drug.
             Examples of live vaccines include, but are not limited to, the following: measles,
             mumps, rubella, varicella/zoster (chicken pox), yellow fever, rabies, Bacillus
             Calmette-Guérin (BCG), and typhoid vaccine. Seasonal influenza vaccines for injection
             are generally killed virus vaccines and are allowed; however, intranasal influenza
             vaccines (eg, FluMist®) are live attenuated vaccines and are not allowed.