B‑lymphocytes are cells of the immune system responsible for the antibody‑ mediated immune response. As estimated, a human body can produce as much as 1011 specific antibodies. There are no specific genes coding for every individual antibody in the human genome. Discrepancy between the huge diversity of antibodies and limited coding capacity of the genome is solved by combination of unique arrangement of genetic information for immunoglobulin and unique genetic and somatic processes providing this wide spectrum of antibodies. On one side, these mechanisms represent a life protecting source of a wide spectrum of antibodies but at the same time, they can be life threatening by raising the risk of a serious tumor disease, the B‑ cell lymphoma. Double‑hit lymphomas represent a specific group of B‑ cell lymphomas often featuring concurrent rearrangements of BCL2 and MYC genes. Activation of the MYC oncogene, typical for Burkitt lymphoma (BL), causes strong stimulation of cell proliferation. High activity of BCL‑ 2, typical for follicular lymphoma, induces resistance to apoptosis. Concurrent damage of regulation of apoptosis and proliferation is probably responsible for the typical clinical manifestation of double‑hit lymphomas – aggressive course, resistance to conventional chemotherapy, high-risk of early relapse, short overall survival, frequent extranodal and central nervous system involvement. Recently, these lymphomas have attracted a strong attention of researchers as they provide sharp insights into processes of lymphocytes maturing and lymphomas development and highlight the double‑edged nature of mechanisms allowing the antibody broad diversity. Case report: Fifty‑ three‑year‑ old man was diagnosed with B‑ cell lymphoma unclassifiable with features intermediate between diffuse large B‑ cell lymphoma (DLBCL) and BL, based on morphology and immunophenotype. Fluorescent in situ hybridization analysis revealed double‑hit lymphoma diagnosis as the tumor cells bear t(14;18) translocation concurrently with the MYC gene rearrangement. The patient died five months after diagnosis.