Concentrations of the complement systems’ components in our tests were adjusted within the ranges of healthy adults (Fig. 4a) Our measurements revealed that increasing the concentrations of the C1q and C3 resulted in the statistically significant increase in the efficacy of the HER-2+ SK-BR-3 and the patients’ HER-2+ breast cancer cells’ killing by trastuzumab and anti-HER-2 antibodies as compared to labeling with the isotype antibodies or labeling of the HER-2− MCF-7 cells. Here, C3 is linked to breast cancer.