Richter transformation (RT) represents a rare but highly lethal evolution of chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), most frequently manifesting as diffuse large B-cell lymphoma (DLBCL). Despite therapeutic advances in CLL, DLBCL-RT remains characterized by rapid progression, profound treatment refractoriness, and short survival with conventional chemoimmunotherapy, underscoring the need for a refined biological and therapeutic framework. A defining feature of RT is clonal relatedness: most cases arise through linear or branched evolution of the antecedent CLL clone and carry an inferior prognosis compared with clonally unrelated cases that resemble de novo DLBCL. Recent multi-omic data further indicate that clonally related RT commonly originates from minute, transformation-primed subclones detectable years before clinical emergence, shifting RT from a late stochastic event to an early-established evolutionary trajectory. At transformation, recurrent genetic lesions of TP53, CDKN2A/B, NOTCH1, and MYC cooperate with B-cell receptor-associated programs, epigenetic reconfiguration, and metabolic rewiring toward OXPHOS- and mTOR-driven states, collectively promoting genomic instability and aggressive growth. In parallel, RT develops within a profoundly immunosuppressive microenvironment marked by PD-1-expressing malignant B cells, PD-L1-rich myeloid niches, exhausted T cells, expanded regulatory T cells, and M2-skewed macrophages interconnected by redundant checkpoint and cytokine networks. Therapeutic strategies are rapidly evolving, including pathway inhibitors, immune checkpoint blockade, T-cell-engaging bispecific antibodies, CAR-T therapies, and antibody–drug conjugates. This review integrates current insights into RT pathogenesis, immune escape, and emerging therapies, highlighting opportunities for biomarker-driven patient stratification, rational combinations, and earlier interception of transformation-prone disease.

Background and Clinical Significance: Mastocytosis and mast cell activation syndrome (MCAS) include conditions in which patients manifest signs, symptoms, and laboratory findings consistent with mast cell activation and can only be diagnosed in the presence of specific criteria. Mutations of ZRSR2, a gene involved in RNA splicing, are not closely associated with mast cell disorders, but rather with myelodysplastic syndromes development. Case Presentation: We report a case of a 37-year-old man who was referred to our institution for anaphylaxis after a bee sting and elevated serum tryptase levels (17.8 ng/mL in the first sample and 19.2 ng/mL in the second sample). Complete blood count was unremarkable. Bone marrow biopsy showed signs of dysplasia and some CD25+ mast cells. ASO-qPCR and targeted myeloid NGS analysis did not detect the KIT p.D816V mutation, but rather showed the presence of a pathogenetic variant of the ZRSR2 gene (p.S447_R448del) with a variant allele frequency of 7.4%. Mastocytosis could not be diagnosed based on the established diagnostic criteria. The patient’s symptoms were not recurrent and tryptase release was not event-related; therefore, a diagnosis of MCAS could not be made either. Taken together, these findings led to the diagnosis of clonal hematopoiesis of indeterminate potential (CHIP). A watch and wait strategy consisting of clinical evaluations, blood tests, and cardiovascular risk assessment was initiated. Conclusions: This case report highlights the importance of combining clinical and laboratory findings, hematopathology, and molecular analyses to establish the most probable diagnosis in challenging cases. It also underscores the possible relevance of identifying predisposing conditions, such as CHIP, in order to guide counseling and follow-up strategy.