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Background Recent clinical and molecular studies of Asian breast cancer suggest that the immune microenvironment of Asian breast cancer patients may be more active compared to patients from Western cohorts, potentially leading to better clinical outcomes and better response to immunotherapy. However, the causal factors underlying this difference between populations are not well understood.Methods Using genomic, transcriptomic, and digital pathology data from a cohort of 934 Malaysian breast cancer patients, we examined the relationship between transcriptomic immune scores and spatial immunophenotypes with patient prognosis in our cohort. Using these data, we also investigated the association of immune scores with germline polymorphisms, the tumour microbiome, and lifestyle risk factors.Results We found that immune-associated gene expression signatures and spatial immunophenotypes in our Malaysian cohort were associated with clinical outcomes such as progression free survival and overall survival, particularly signatures for B-cells, T-helper cells, and lymph vessel development. We also found small but significant associations between immune scores and germline polymorphisms, the tumour microbiome, parity, and breastfeeding.Conclusions Our results are consistent with a combinatorial model where the tumour immune microenvironment of Asian breast cancer patients is influenced by multiple genetic and environmental factors, each with small effect sizes. Our results also highlight the complexity of the immune response to breast cancer and demonstrate the need for population-specific studies in cancer immunology.Ethics Approval Patient recruitment and sample collection was reviewed and approved by the Independent Ethics Committee, Ramsay Sime Darby Health Care (Reference nos: 201109.4 and 201208.1), as well as the Medical Ethics Committee of the University Malaya Medical Centre (Reference no: 842.9). Written informed consent to participation in research was given by each individual patient.