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B-cell acute lymphoblastic leukaemia (B-ALL) harbouring immunoglobulin heavy chain (IGH) gene rearrangements encompasses a clinically and molecularly heterogeneous group of haematological malignancies. Among these, the subset presenting with hypereosinophilia represents a particularly underrecognised diagnostic challenge, frequently resulting in significant delays to appropriate treatment. The paradigmatic example is B-ALL with t(5;14)(q31.1;q32.3)/IGH::IL3, in which dysregulated interleukin-3 (IL-3) expression driven by the IGH enhancer leads to pronounced reactive eosinophilia that may entirely dominate the clinical picture. The leukaemic blast burden at presentation may be deceptively low, and the eosinophilia—reactive rather than clonal—can divert clinical attention towards primary eosinophilic disorders, myeloid neoplasms with eosinophilia, or common reactive causes including parasitic infection and atopy. Beyond the IGH::IL3 fusion, other IGH partner genes, including CRLF2, EPOR, and DUX4, create signalling environments that may further potentiate eosinophil recruitment and bone marrow expansion. Accurate diagnosis requires the integration of peripheral blood film morphology, multiparameter flow cytometric immunophenotyping, conventional cytogenetics, fluorescence in situ hybridisation (FISH), and, increasingly, next-generation sequencing (NGS) or transcriptomic profiling. Treatment of the underlying leukaemia with standard multi-agent B-ALL chemotherapy regimens remains the definitive approach, resolving the associated eosinophilia as the leukaemic clone is suppressed. Emerging immunotherapeutic agents and targeted kinase inhibitors are reshaping the treatment landscape for molecularly defined B-ALL subtypes. This comprehensive review synthesises current knowledge of the molecular pathogenesis, clinical presentation, diagnostic challenges, and therapeutic strategies for this underrecognised entity, with the dual aim of heightening clinical awareness and reducing diagnostic delay.