Aims: To unravel the cause and possible prevention of breast cancer, five potential polysaccharides from guava seed (GSPS), common buckwheat (CBPS), bitter buckwheat (BBPS), red Formosa lambsquarters (RFLPS) and yellow Formosa lambsquarters (YFLPS) were selected to treat human breast cancer MCF-7 cells with direct action or indirect tumor immunotherapy using polysaccharide-treated splenocyte-conditioned media (SCM). Methods: Five different polysaccharides were partially characterized. Cytokine levels produced by type 1 helper T-cells [Th1; interleukin (IL)-2 and interferon-γ] and type 2 helper T-cells (Th2; IL-4, IL-5 and IL-10) in SCM were determined using ELISA. Breast cancer MCF-7 cell viability was determined using an MTT assay. The association between cytokine levels in SCM and the corresponding breast cancer MCF-7 cell viability was investigated using the Pearson product-moment correlation coefficient. Results: Total carbohydrate and protein contents indicated that these isolated polysaccharides might be proteopolysaccharides or glycoproteins. Molecular weights of purified polysaccharides fractions from the five isolated polysaccharides were determined. All 5 selected crude polysaccharides, in particular GSPS, BBPS and RFLPS, significantly increased IL-10/IL-2 cytokine secretion ratios by splenocytes. Treatment with SCM prepared in the presence or absence of the polysaccharides, but not the direct action of the polysaccharides, significantly inhibited MCF-7 cell viability. GSPS-, BBPS- and RFLPS-treated SCM demonstrated slightly enhanced inhibitory effects against breast cancer MCF-7 cell viability. There were negative correlations between MCF-7 cell viabilities and IL-10 cytokine secretion levels, as well as IL-10/IL-2 cytokine secretion ratios in the corresponding SCM. The present findings suggested that an increased Th2/Th1 cytokine secretion ratio, particularly an increase in IL-10, by immune cells including T lymphocytes, may inhibit MCF-7 cell growth in the tumor microenvironment. GSPS, BBPS and RFLPS may be further applied to treat human breast cancer via tumor immunotherapy by increasing the Th2/ Th1 cytokine secretion ratio.