Primary effusion lymphoma (PEL) is classified as a diffuse large B cell lymphoma. It is a rare malignancy of plasmablastic cells that occurs in individuals that are infected with the Kaposi's sarcoma-associated herpesvirus (i.e. KSHV/HHV8). Plasmablasts are immature plasma cells, i.e. lymphocytes of the B-cell type that have differentiated into plasmablasts but because of their malignant nature do not differentiate into mature plasma cells but rather proliferate excessively and thereby cause life-threatening disease. In PEL, the proliferating plasmablastoid cells commonly accumulate within body cavities to produce effusions (i.e. accumulations of fluid), primarily in the pleural, pericardial, or peritoneal cavities, without forming a contiguous tumor mass. In rare cases of these cavitary forms of PEL, the effusions develop in joints, the epidural space surrounding the brain and spinal cord, and underneath the capsule (i.e. tightly woven collagen fibers) which forms around breast implants. Less frequently, individuals present with extracavitary primary effusion lymphomas, i.e., solid tumor masses not accompanied by effusions. The extracavitary tumors may develop in lymph nodes, bone, bone marrow, the gastrointestinal tract, skin, spleen, liver, lungs, central nervous system, testes, paranasal sinuses, muscle, and, rarely, inside the vasculature and sinuses of lymph nodes. As their disease progresses, however, individuals with the classical effusion-form of PEL may develop extracavitary tumors and individuals with extracavitary PEL may develop cavitary effusions. PEL typically occurs in individuals who are immunocompromised, i.e., individuals whose immune system is weakened and therefore less able to fight infectious agents and cancers. This weakening is ascribed to KSHV/HHV8 infection that is commonly further promoted by concurrent human immunodeficiency virus (i.e. HIV) infection, prior organ transplantation, the decline in immunity that develops with aging, and/or cirrhosis of the liver due to hepatitis B or C virus. The plasmacytoid cells in PEL are also commonly infected with the Epstein-Barr virus (i.e. EBV). EBV is a known cause of various Epstein-Barr virus-associated lymphoproliferative diseases including various B-cell lymphomas. However, the role of this virus in the development of PEL is not clear, although some studies suggest that EBV infection cooperates with KSHV/HHV8 infection to promote the development and/or progression of this disease. Formally, PEL is defined by the World Health Organization, 2016 as a KSHV/HHV8-positive and KSHV/HHV8-driven large B-cell lymphoma. This lymphoma also belongs to a group of lymphoid neoplasms with plasmablastic differentiation that involve malignant plasmablasts but differ from PEL in the types of tissues where they accumulate, the gene