BackgroundBreast cancer (BC) is a highly heterogeneous disease, and although immunotherapy has recently increased patient survival in a number of solid and hematologic malignancies, most BC subtypes respond poorly to immune checkpoint blockade therapy (ICB). B cells, particularly those that congregate in tertiary lymphoid structures (TLS), play a significant role in antitumour immunity. However, Bâcell heterogeneity at singleâcell resolution and its clinical significance with TLS in BC need to be explored further.MethodsPrimary tumour lesions and surrounding normal tissues were taken from 14 BC patients, totaling 124,587 cells, for singleâcell transcriptome sequencing and bioinformatics analysis.ResultsBased on the usual markers, the singleâcell transcriptome profiles were classified into various clusters. A thorough singleâcell study was conducted with a focus on tumourâinfiltrating B cells (TILâB) and tumourâassociated neutrophils (TAN). TILâB was divided into five clusters, and unusual cell types, such as follicular B cells, which are strongly related to immunotherapy efficacy, were identified. In BC, TAN and TILâB infiltration are positively correlated, and at the same time, compared with TLSâhigh, TAN and TILâB in TLSâlow group are significantly positively correlated.ConclusionsIn conclusion, our study highlights the heterogeneity of B cells in BC, explains how B cells and TLS contribute significantly to antitumour immunity at both the singleâcell and clinical level, and offers a straightforward marker for TLS called CD23. These results will offer more pertinent information on the applicability and effectiveness of tumour immunotherapy for BC.